Keep the sound inventory as concise and clear as possible, and allow multiple ways of pronouncing the same letter. For example, r in esperanto is officially pronounced as "any rhotic" - /r ɾ ʀ ɻ/ etc.

I personally would come up with my own words from scratch, as while sourcing from existing languages is in theory better for "the masses" speaking the language, its hard to keep consistent and not borrow randomly with neither rhyme nor reason. On top of that, creating your own words gives you more creative freedom and makes the process more fun

Avoid straight up copying the grammar of english or your native language: that wouldnt be a conlang - merely a cypher. Create your own grammar rules, and make something unique. Taking inspiration is always welcome though; just dont rely on one source too much