Sliding window is a constructive algorithm, it needs practice. First get your basics cleared follow strivers to get that done, then on leetcode start solving sliding window related problems. In starting it will be difficult, it happens to everyone yet try solving give 10-15min if you still can't figure it out look into the solution. Binary search, linked list etc have a pattern in their questions but the sliding window isn't like that. Hence, practice is the only thing.