There are a few things that are basic requirments (for me) before doing giants. (This assumes they have a 3/4 giant on their own, good or bad)
1) A good tap swing.
2) A back extension roll to handstand (preferrably with straight arms)
3) Candlestick
4) Body Shaping/Handstand (for hollow)
After the tap swing motion is established, start with back extension rolls. If they can get to handstands, get yourself a floor bar and a wedge mat, a floor bar and an 8" mat, and a spotter.
station 1: Cast to handstand. Start in a push-up position on the bar. Dip hips to bar, and "pop" into a straddle cast, to handstand. Fall hollow onto the 8" mat. Make sure their shoulders stay in front of the bar until the handstand. Make sure their body hits the mat solid in 1 piece, not chest first, not toes first. If you would like, you can build the "straddle" in for taller girls, or you can have them hold a slight pike to avoid clipping the bar, but they may end up landing on their toes at this point and can hurt more than the bar. Preferrably, if they need to get away from the bar, learn this phase with a straddle.
station 2: With a spotter, lay on back with shoulders at the point the wedge mat meets the floor. Hands should be on a floor bar, extended above as if swinging. Have the girls focus on doing a "piked" back extension roll to handstand, keeping their head neutral until they feel pressure on their hands. This will help them learn to shift properly, and keep a nice hollow, tight body line. They will need a lot of spot (generally) at first, but over time the girls could (possibly should) be able to do this drill on their own. When they feel comfortable with this, and you are satisfied with their progress, move to the next part.
Part 2:
The low bar:
Many coaches will not use this. I do this because I teach an accelerated program due to time restraints in a highschool. It does emphasize poor technique at one point, however, it builds confidence. You can feel free to skip any portion of this, and I will try to explain my logic for each section.
Step 1: Sole circle to stand up. This will teach them how to shift properly, and get them used to going in a circle motion.
Step 2: Many sole circles in a row at varying speeds. The faster the better.
Step 3: Get 2 spotting blocks and place them and build a "channel" behind the low bar. Use this to give them comfort in attempting to cast to handstand, or cast as high as they can. From here, they can work on bail to low bar giant. They will have to bend their legs (hence, the bad form/technique). I like this part because it emphasizes casting to handstand on a bar from a support, and it builds into giants by themselves. Of course, I hand spot this, using the upper arm and hips for the cast, and the wrist/hips for the giant motion. In between turns, work candle-sticks holding onto something solid, like the leg of a beam, or a pommel horse leg. Whatever vertical object that is available.
You will generally get a lot of people tapping early at this point. If you are not worried about them peeling off, you can focus on getting a little above the bar (using a spotting block of your own, or some stacked mats), and spot from their hips with both hands carrying them over. If they are going short, they can pirouette out, if they go over - tada! Giant. Even if they are slightly hollow over the top, if they clear circle it, they have made it over successfully! They just need to learn to open the shoulders at this point. This low bar work really builds comfort with the circling skill, and going upside down, and makes things a lot easier for the spotter since they are not so high up, and there is less potential for gymnast injury.
Part 3: Straps
At this point, gymasts can usually head over the the strap bar for high bar work. They may still want a hand spot (from their wrist and hips), and this is ok. However, I would encourage them to swing on their own and feel out the skill. Re-emphasize straddling or piking for the low bar here as well.
As conditioning, I do a LOT of handstands on a floor bar, as well as pirouettes on the bar, both to handstands and to bails, as well as to their feet. The more they can unerstand how to get out of danger, the better off they will be when they get into a position they are not used to. Further, it will help with hand work later on when they need to start working on it.
That's my progression. I have done it as fast as 1-2 days (a 2 hour session, having already learned a good tap swing), and as long as a month (or longer, if they dont have a tap swing). Generally, they get to the point of hand-spotted giants quickly, but have a rough time moving to the independent work on straps. Give it time, and be patient, and they will get it.
Good luck! I would love to read some other people's suggestions too...