Most cases are simply used where English would use prepositions. In Finnish those are postfixes instead, a bunch of letters tacked onto the end of a word. It’s a case and not a word because there’s no space between the two, that’s it.
Eg “talo” means house. Talossa means “in the house”. Talon means “of the house” (actually, “the house’s” - omg English has cases too, super difficult). Talolla means “on (top of) the house”. That’s not harder than prepositions is it?
I did cheat a bit, because with some words you first got to find the root before you can tack on “-ssa”. The root of talo is also talo, but for some words you got to apply a (simple, purely letter-based) rule. Eg the root of “ankka” (duck) is “anka” so “the duck’s house” becomes “ankan talo”. There’s a bunch of rules to find the root of a noun and you can learn them in half an hour or so.
There’s plenty stuff that’s harder about Finnish (notably the vocabulary), but the cases are peanuts.