How do I easily get thin slices of guava paste?
Guava paste is easy to use when making it fresh, but canned with pectin it can be a pain.

You probably need to go warmer or colder to get it to do what you want.

If you go warmer you could try a couple of tablespoons on a small plate in the microwave. I usually put a coffee cup of water next to it to slow down the heating process. Try 20 seconds at a time until you could use a knife to smear it. If it has too much pectin or is dried out this might not work.

If you go colder you could try cooling it down in the freezer 5-10 minutes at a time. Use a wire cheese slicer (or a piece of unflavored dental floss) and try shaving off smaller slices off your block. Use a fork to lay the cold thin sliced guava paste on your bars and repeat.

You can also look for a different recipe that lets you use thicker slices of guava paste - there are several good muffin recipes where the molten guava in the center is most excellent.