Edit: For kanji, make sure you tap/click "How to use" and carefully read the instructions.
For words, make sure you read "How to add dictionary links", and be careful with the "Type" and "Privacy" settings.
PS: Public kanji mnemonics and usage notes aren't visible to other users until approved. You usually know when your note/mnemonic is approved when it gets 1 "like". If it stays at 0 for a week, it probably won't get approved. Approval seems to happen in bulk, 1-2 times a week.
That's not important if you're just writing them for private use though.