Know your vocabulary according to what you're writing about. If unsure or at a loss for words, describe the word. (Avoid this as much as possible)
Prepositional phrases
In: in time, in defense of, in good condition, in general, in honor of,
At: at risk, at a discount, at all costs, at night, at peace,
By: by law, by definition, by all means, by chance, by accident
Synonyms
Happy → jolly, merry, content, cheerful, joyful
Good → of high quality, satisfactory, in order, fine, up to standard
Avoid words such as → instead use
Like, things, stuff, nice, about → such as, specific object, awesome, approximately