That is one of the most often used words in the English language and one of the most versatile. It can be a pronoun (That is my belt.), an adjective (That book is my favorite.), an adverb (The worm is about that long.), and a conjunction (I was so tired that I fell asleep standing up.). That is one busy word.
Many times that can be cut from the sentence without sacrificing clarity. Closely evaluate all instances of that, especially those following a noun, e.g., I loved the casserole that you made.
The best way to determine the necessity of that is to read your sentence aloud, first with that and then without it. Listen to the sound of the words. Does the sentence still make sense without that? If so, cut it. I loved the casserole you made. If the clarity is compromised without it, leave it in. I need your signature on that document I sent yesterday.
Try this. Go through a piece you wrote recently. Circle all instances of that. Then read each sentence aloud, both with and without that, and determine whether it stays or goes. Let us know what you find. Happy editing.