u/East-Toe-475

when i download a rom it downloads as a zip file, but i have no idea what to do after that?? i tried extracting it and i tried watching youtube videos but i think im just dumb. i already have twilight menu and i successfully got a game in there that i play on my ds lite but its been months since then and i already forgot how to do it. i know it has to be in a NDS file but im so confused how to get it.