how do i install the mods
If the mods have a resources folder you should be able to just add them into the one in the game, but if not you just add the various things from the mod into the correct places.
If it's an enemy mod you put the mod's json into the enemy json folder, you put their sprites folder into the sprites folder - not the individual sprites, just the folder with the sprites in it - and you put their sounds in the sound folder.
I believe if it's a player character mod you do generally the same thing - json into the player json, etc... - but you also have to replace everything, so replacing japes' current jason and sprites with the new character's stuff.
If you want to also do things like effect how likely an enemy is to spawn in certain zones there is also a Modding Resources folder that has a guide in there.