I'm working on this right now. I should have been taking before and during pictures, but haven't been. Lots of seat time. I'm fixing a lot of erosion issues first, so my efforts won't be in vain. That is the primary thing to do. Make sure your drainage issues are correct and you have a plan for any runoff, then prep for sod.
I've cut a swale probably 250 ft long along my property line to catch runoff and direct it to my main drainage ditch. Then I'm redoing the main drainage ditch with my FEL and box blade and fixing a lot of erosion spots and smoothing everything out.
For the sod, I'm getting Bemuda and will start with sodding the main drainage ditch and the side swale and side yard will be seeded and covered with straw matting.
Pictures would help, but to answer your question, you need a smooth surface for the sod. You can round up the existing grass and weeds if you like the lay. The sod comes with top soil. Better option is to lightly till the area before applying the sod and if you have a roller, rolling it one time would help make the contact. The most important part is to water water water after laying it for a good week or more so the roots can take hold. The areas I'm not sodding, but seeding instead (my side property), I'm prepping with putting top soil an inch or two thick and smoothing before seeding, then laying straw mat to control run-off, then watering well also. In my parts, a lot of lime helps and a little starter fertilizer.
