Not much. Bermuda seed sucks you should do sod instead.

The best thing to do is to pull them, I know that probably isnt what you wanted to hear.Crabgrass is easy to pull when they are young plants.The longer you let them grow they will crowd out the bermuda and basically stunt the bermuda's growth.I am in the same situation as you but I am spriging instead of seeding...The bermuda is doing great and spreading but I got alot of crab grass getting in the way. So I will pull them and throw them in a 5 gallon bucket.Its a little time consuming but pays off..You can also buy the herbicide that kills the crab grass but doesnt harm youre bermuda.I personally wont never use that stuff again.Although it doesnt kill the bermuda, mine looked bad for a good 4 weeks(turned to a ugly green, almost yellow)..I wont never use a herbicice again...Probably should have put down a pre emergence after you did the roundup.Thats the best way to deal with crab grass.

EDIT: Nevermind about the pre-emergence idea, forgot you was seeding

I know you have already jumped into this pool, but here's what you should have done. Maybe this will help the next guy.

1. Water lightly, 3x per day, for a week. This will germinate all the crabgrass seed in the yard.

2. Spray with RoundUp to kill everything.

3. Continue watering for another week to sprout other weeds.

4. Spray with RoundUp to kill the rest of the weed seeds that sprouted.

5. Rake up all that dead stuff.

6. Level the surface to perfection.

7. Seed.

8. Roll the seed down with a water fillable roller or walk on every square inch of it.

9. Continue watering to sprout the new grass seed.

10. Mow new bermuda when it gets up to 2 inches. Mow it back to your mower's lowest setting.