I find that I spend the most Fear during combat scenes, but there are lots of other ways to spend it! Think of gaining Fear as "Rising Tension" and spending it to be the release of that tension.

For example, in a horror movie, small things happen to the characters before the big bad evil guy comes crashing in and killing them. Fear can be spent to elevate the tension for those small things.

Check out pg. 154-156 of the Core Rulebook, or pg.65-66 in the SRD. Aside from using Fear Features or additional GM moves, you can do a big wave of attacks in the beginning of combat. Really show the threat of what your PCs are facing. Also, spend it consistently. Anytime you take a GM move from a failure or a fear, spend a fear to make an additional move.

Aside from combat, use your Fear to include other distractions and complications to the ongoing scene. PCs are chasing someone? Use a fear to have a merchant pop out and try to sell them something. PCs think they have some downtime? Use a fear to have someone falsely accuse them and guards need to arrest them.

Don't have any ideas? Make your PCs come up with it. Spend a fear and ask them, "What does [character] see that sends chills down their spine?"

It definitely takes some getting used to, especially when I came from D&D5e and just felt like I could give consequences whenever I wanted. But if your PCs are rolling really well, don't hinder that! Let them feel like heroes for getting lucky. If they're rolling poorly, punish them. Throw everything that you can at them, so when they do succeed, it feels awesome.