The easiest piece of advice:
When you decide upon a move, before playing it ensure that ALL of your opponent's (forcing moves) in response to your move, i.e.
- Checks,
- Captures,
- Threats.
Can be met by you safely via careful anaylsis, as opposed to "hoping" you can deal with it later (after he moves!).
If there's something he can play that you can't meet safely, your intended move is clearly wrong come up with something else to play.
The vast majority of chess players out there simply ignore this out of cluelessness or arrogantly think "of course I do this" even though they don't do it consistently on every move.
If you ever wondered what separates the strong players from the weak ones, the thought process above would be one of the big differentiators.