Lawns - Removing dead grass?

I am removing my St. Augustine lawn. I put down round up about 2 weks ago. The lawn is mostly straw like now. My question is, how do I get all the dead grass up so i can seed my new lawn. Can I just rototil the dead grass in or should i remove it?

You could rototill it in but if there is any viable plant material left it will come back and contaminate the new lawn. I would probably rent a sod cutter and cut it out and disguard the cut material or get it scraped off with a box blade.
What type of seed do you plan to re-establish the lawn with? That info could help with the answer to your question.

I plan to seed with a fescue/ kentucky bluegrass mixture.

And one thing I found when redoing my lawn back in VA...it's tough to get a nice even surface if you leave the old stuff there. It will break up in clumps that the tiller just won't tear into much smaller pieces. Then you have to go back and rake and rake to get them out.

Rent a sod cutter, roll it up in sections small enough to handle and dispose of it. Add fresh topsoil if required.

If you are positive you got a complete kill you could scalp the st augustine down as low as possible and remove the cuttings. Then you could just seed right into the leftover plant material using it as a mulch. Again, I would only consider this if I were positive that I had it all killed out!
I'm not sure about the summer temps in your area, but getting cool season grasses to germinate this time of year is impossible around here. We do it in the spring or fall.

I don't plan on seeding until late September, so time is on my side. I've used round up 3 times, about 7 days apart. And everything is yellow now, but still very thick.
Also after till the soil, can i add steer manure or do i need a topsoil and after i put the seed down do i need to put something over the seed so they don't dry out. A thin layer of topsoil, maybe?

I would stay away from manure unless it is 100% composted as it will be full of weed seeds.

What do you recommend to put over the seed shen i lay it down? Anything?
Also, if I'm not laing the seed until late September, can i put the steer manure down and see if any weeds come, then kill them?

You could have a mix of cool season and warm season seeds in the manure and not know it till the fall. My opinion is to never use raw manure. As far as covering the seeds goes, the more soil to seed contact the better but seeds will germinate on concrete if the get enough moisture.

Do I need to remove all the roots even thought he blades were dead. i have been pulling the grass up and I notice little string like roots still there. Will they invade my newly seeded lawn? Will another round up treatment work?

Any plant that has round up sprayed on it and was killed by that application has dead roots as well.Also spraying round up on roots will not accomplish anything.round up essentially becomes nonfunctional once it contacts the soil and is designed to be absorbed by leaf surfaces.I'd get what can easily be removed and till the rest in.