Don’t write “that” if you mean “who” or “which” or you just don’t need a word at all.
Writing about people “who” rather than people “that” is one little change that can humanize your writing and help you connect with more people.
People “who;” things “that.” Easy to remember.
Write “which” instead of “that” if you are distinguishing one from many.
If you’re not sure, just repeat to yourself: Which witch is which? If your”which” works the same way, then use it.
Don’t use “that” if you can remove it from your sentence without making it more difficult to understand.
You use “that” more in talking than you need to in writing, so it often slips in. So nuking “that” is often an easy to reduce your character count, handy for Twitter and other places you want to say more with less.