Beginner woodworker here, when I initially posted my plans for cabinets that I'm building to surround my washer and dryer a number of you suggested I build a separate base for the lower cabinets as it would be far easier to level on my likely very uneven floor. I want to heed that advice and build the separate base but am not finding much information on how I should actually construct the base. I was thinking some 2x4s in a ladder type arrangement but I don't know if that is best. Is there reference material that I can look at for construction? These cabinets are fairly large and heavy so I want to make sure I construct this properly.

Also, once I construct the base, am I leveling it with shims like I would with the cabinets against the wall, or is there a better way to level it? Thanks so much in advance on this. You all have been instrumental on me inching forward with this project.