To achieve a level surface for your DIY tile installation, you can use self-leveling compound. Ensure the floor is thoroughly cleaned and primed before adding the compound. Seal any holes and seams in the plywood with caulk to prevent the liquid leveler from escaping. Install sill sealer at the walls to prevent the liquid underlayment from escaping. Use cardboard to isolate large holes like heating vents and floor drains before pouring the leveler. Dam up doorways with wood strips and seal with caulk for a tight seal. Follow the manufacturer's instructions for mixing and applying the self-leveling compound.