In Spanish is there a good way to say “just”? For example, “How do I fix this?”, “Just do this.”

Answer 1

You can use "solo" or "solamente".

So going off of your example

¿Cómo arreglo este?

Solo haces esto...

Both are adverbs, but solamente is used more to mean only, so if you're saying

I just want a taco, as in I ONLY want a taco, you would use "solamente quiero un taco"