When you are placing rebar on a slab is very important that you know the following:
1. first you want to know the exact location of your walls. remember when you are building a slab you need to place your plubing and first. it is important that you know the location of the walls so you can properly place all your stubs and penetrations.
2. locate all your structural shear walls and hold downs. this is important as all these items must be in place prior to placing rebar. the other aspect is that you dont want these items to interfere with your rebar layout.
3. once you find the location of all these items, you can start placing your rebar. lay out your rebar by marking the spacing on the ground and then placing your bars. tie them together after you have place them in the right place.