How to make a straight line up against the trim with your wall paint.
Start by painting the trim first . You can pass over onto the wall by an inch or so.
Loading up your 3" brush full with paint helps you to 'find your line'. With subtle pressure, run the tips of the bristles to the corner. You will find the right resistance to your bristles in the corner to make the line straight.
To make the line even straighter, take your 5-in-1 tool wrapped with a wet rag and drag it down the corner. At just the right angle, this will give you a crisp, straight line.
A 5-in-1 tool and clean, wet rag can give you a laser straight line. That is if you don't drink too much coffee!