^^^ good advice up above. You can get a pack of disposable aluminum cookie sheets to use until she gets big enough for higher sided pans.
I do have one thing to add. Sweep up her little messes from the floor and put them in her flat sided litter boxes so she knows what they are for.
For pee: sprinkle some litter over it, once the litter has absorbed the liquid, sweep it up and put it in the litter tray
For poop, just pick it up and put it in the litter box.
Clean soiled areas with an enzymatic cleaner to remove all traces of odor, to avoid confusing her. If she smells her waste on the floor, she may think it is okay to go there again.
As stated above, make sure there are pans near by in any room she is in. She's only a baby and may not realize she has to go until it's too late to travel from one room to another to find the litter box.
Be sure to not use clumping litter until she is at least six months old.
Avoid litter that has perfumes in it.
Plain clay litter, or if you don't like clay, pine pellet litter.