In my opinion, the most difficult part about learning Esperanto is tackling all the correlatives. They are quite logical in their construction (as is all of Esperanto!), and you could easily form them if you had a small, pocket-sized table on you. But, there are still a good number of them, and sometimes they can be difficult to recall because of their similarity.
One trick I’ve used to help me remember the ki- correlatives is to place them with another word, to help with the question you’re trying to ask. By recalling these compound words, you can more easily remember the necessary correlative, as well as have a quick and useful phrase memorized. The two I use most often (and the only two I can recall off the top of my head right now) are “kiudirekte” and “kiamaniere.”
Kiudirekte estas la hospitalo? – Which way is the hospital?
Kiamaniere gxin vi faris? – How (what kind of way) did you do it?
Not only are these words nice and specific – they also can help you recall the ki- correlatives. “Kiu” more or less means “which one,” which we can easily deduce from “kiudirekte,” which at a glance might make you think “which direction.” Similarly, “kiamaniere” brings to mind “what kind of” and “manner.”
Maybe this trick will help you remember your correlatives, too!