You don't need to remember 300 openings. Remembering openings is the most straight forward thing to study - you just need to remember them and then select in game as your opponent gets a choice as to which of those 300 you are playing as well. That is hard to do in practice, but it is easy to explain.
For most people you are better off ignoring all that though. Instead you should train tactics and endgames. Both are easy enough to explain, but it isn't useful to just memorize them. You have to see when they apply even though there are other unrelated pieces on the board. You have to see when they apply even though the pieces are in slightly different positions from what you studied. You have to see how they will apply after complex sequence of moves even though they don't apply yet.
For most people you are better off ignoring all that though. Instead you should train tactics and endgames. Both are easy enough to explain, but it isn't useful to just memorize them. You have to see when they apply even though there are other unrelated pieces on the board. You have to see when they apply even though the pieces are in slightly different positions from what you studied. You have to see how they will apply after complex sequence of moves even though they don't apply yet.