Break down kanji by parts. Use mnemonics. Only worry about most common readings.
For mnemonics I like to incorporate the meaning, drawing, and readings of the kanji. Remembering one part helps trigger the memory for the rest. It's good to associate it with a word that uses the kanji too.
Often, the onyomi reading comes from one of the parts of the kanji. When those parts appear in other kanji you can guess that the reading corresponds to that part. This technique can be extended even further by using Cure Dolly's Sounds Sisters https://learnjapaneseonline.in... (I don't use the Sound Sister's but many find it useful)
This is a pretty simple one, but try to study the words with the kanji or at least read the sentences with them, I find that they stick in my mind much better that way. If you know or at least can think of some words with the kanji, the onyomi and kunyomi become easier as well, since at least, renshuu gives you multiple choice questions and you can use the knowledge of the vocab to your advantage! I also recommend using something like the online dictionary Jisho - https://jisho.org/ when learning them, if you get stuck. Some people may consider it "cheating" but in my opinion it's better than just mindlessly guessing the meanings and maybe getting it right and learning nearly nothing... Using Jisho has helped me to memorize both the meanings and the yomis, so give it a try if you can! Writing it down on paper with all the info can also help, but it's pretty boring to write down all the kanji, so it's very efficient for some people and not others, so you can find what fits best for you! :) Hope this helps!
This best tip I can offer is to give yourself permission to make mistakes. There are a lot of kanji, most of them have multiple readings, some of them have multiple meanings, and an awful lot of them are homonyms. It’s a big task to learn them all, and it won’t happen overnight. Japanese elementary school students take years to learn all the N3/N4 kanji.
You can do it, but setbacks are inevitable. Just know that everyone else is fighting the same battle, and don’t give up.
