I've built a few custom homes for individuals who wanted to do their own painting. This is understandable since many people have some experience in this area and its not extremely difficult to learn. It may slow the job down a little because experienced painters can paint faster, but this savings can easily offset this problem. Or you may choose to just paint the walls and hire out the trim. Typically I can do some of both.

Step 1: Primer

Once the drywallers are out of the way, you are ready to paint the walls. Some builders paint all ceilings and some choose to leave them white. I normally do not paint the ceilings unless another color is desired. I use what's called a 'stom & drag' finish which is applied with a mop-like applicator, then taking a trowl and lightly dragging it over the surface. If the ceiling is to be painted, this should be done first. I painted my walls before any trim on the interior doors are installed. I make sure to put plastic over the windows if spraying will be done. I'm not that accomplished with paint sprayers so I just roll the large surfaces. There is no over-spray with this method, less masking and no expensive sprayer to purchase, maintain and clean.

Primer

You'll need to get a good drywall primer and use this for the first coat. These are available in 5 gallon containers. Read on the label as to coverage to determine how much you'll need. Lets say I ordered 7,000 sq ft of drywall for a house with 1,300 sq ft living area and 450 sq ft garage. If I'm only painting the walls, I will deduct 1,300 + 450 (or 1,750) from 7,000 = 5,250 ft to allow for ceilings. If the directions say one gallon covers approximately 400 sq ft then I need 13.1 gallons (5,250 / 400). In this case it would be better to buy 3 five-gallon containers. Better to have a little too much than not enough.
The next thing you will need are six roller pans, pads and applicators with poles. I get a good sturdy pole that telescopes out so it can be adjusted for different heights. A short-handled roller with an 8 in wide pad works best for small areas such as closets. I use 18 in pads and wide roller pans for everything else. They cover twice as much in the same amount of time. I use a light orange peel texture on most walls so I need a pad with a medium nap. They are rated for smooth, medium and rough surfaces. Get what's appropriate. These pads are expensive: $9.00 each for 18 in. so I always reuse them from one day to the next. When you are finished for the day, you simply roll them up in garbage bags to keep the air out of them and they will be fine this way for several days. If you are working with different colors you can label the bags. Once the walls are primed it is necessary to take the brush and get areas that the roller could not reach. Mostly just the corners where the walls meet and around the top where the wall meets the ceiling. Due to the texturing on the ceiling, this line is not always perfect which makes masking difficult, so I use a 24 in long metal or plastic edger to trim this out. Just remember to keep dragging your hip pocket and wipe off the back side of the edger each time you use it because paint will accumulate here.  Be careful not to roll too high when painting the walls or you will get roller marks on the ceiling. If this happens, you can just let it dry before coming back to lightly scrape it away. I usually ask my drywallers to leave a partial bucket of premixed ceiling texture this way I will have a match to use for the touch-up if necessary. If the ceilings are painted, there isn't a problem as you can match the paint so just touch up.

The primer dries over night leaving you ready on day two for wall paint. You will use the same procedure here as when priming the walls. Again, keep a used roller wrapped up in plastic bags, as you will want to come back once the trim work and carpet are finished to touch up any smudges etc.

Staining

I usually do all of my own staining because it is simple to do and goes quickly. Items you will need:
Staining pads (Lowe's carry these in 2 pacs. They are soft and resemble a sponge and cheap at only $2 per pac)
Plastic painter's gloves (they are throw-away and you will need to get a 50 pac because they tear easily)
Lots of rags for wiping stain (soft cotton is best, they sell these too but they are pricy... old t-shirts work great!)
A pair of saw horses
Roll of plastic to cover garage floor. You can use the garage in the house you are building for staining trim and interior doors. Follow directions on the label for ventilation and temperature. Most products require a 50 degree or higher temp. And I leave the garage door open if outside weather permits. If you can not get plenty of ventilation I recommend using a respirator. 

When I am installing my own trim (base, door and window casings) I like to paint or stain and varnish before I install the material. Since I have to go over it all once to fill nail holes, I just touch up and fill in any small gaps with putty that is stained to match.

If you are hiring all painting done then its only a matter of coordinating everything with your painter. They may come in and do all the masking, spraying and painting after all the trim work has been completed.

Staining trim is very simple. I get a piece of 1x12 pine decking that is 16 ft long to place across the saw horses. This gives me a flat surface to lay trim boards on while staining and wiping. Then I find an area where I can build racks using about 10-12 trim boards per row in tiers.

Left over bricks and some 4 ft long 1x2 strips for cross braces work great. This way you can stack 4-5 levels helping to save space. 

When ordering casing for doors I add up my doors and multiply by 4 to get a count. Make sure to add 2 more for the interior sides of any exterior door. These can easily be ordered in 7 ft lengths which works great with minimal waste. The tops may be cut from random lengths. 

Doors 

If you are staining interior doors, I recommend buying a bundle of shims used for installation along with roofing nails. I'll explain why shortly. I find it best to stain both sides of the doors at once because stain has a tendency to run around the edge and onto the backside. If it dries this way without being wiped off, it is difficult to remove. If you find any runs like this later on you can use a small amount of laquer (paint thinner) on a rag to gently wipe off then restain. Look everything over good before you apply varnish. Sanding these areas doesn't work as well because stain doesn't absorb well once the area has been sanded.

 

I  stain one side of the doors, wipe them down and then flip it over and repeat. You'll need a cloth stretched accross the saw horses so that no marks will appear where the door touches the horse. You don't have to wait for them to dry before flipping so long as you've done a good job wiping the excess stain. 

 

Once they are stained and wiped down you are ready to stand them up against a wall to dry. Here's where the shims come in handy. I tack 2 shims about 18" apart into the center of the top edge on each door. Then you can lean the door up against the wall without getting stain on the painted wall. Make sure to double check these doors after you stand them up and before they dry for any runs that may have been missed when lying flat.

 

This rarely happens on smooth solid panel doors but doors with detail like 6 panel doors should be checked carefully. Stain will accumulate in crevices and once the door is stood up you may see some runs.

 

Varnish 

 

I usually hire my painter to come out for a day and spray all my doors and trim. I can't compete with a professional spray job by varnishing myself with a brush. I use a lacquer that dries fast and we can finish the trim in a small house (3 coats) in a day. I provide a couple of helpers to move things around and let my painter focus on spraying.

 

You should prepare the garage walls by hanging plastic from wall to wall, ceiling to floor and also covering the floor. The doors can be leaned against the walls and sprayed in place. The shims allow you to flip the door and do both sides. A hand stapler works well for attaching the plastic to walls.

 

Any other trim materials for closets or shelving should also be done during this stain and varnishing process.