HOW CAN I CUT IN STRAIGHTER LINES AROUND EDGES AND CORNERS WHEN I PAINT?

Try putting more paint on the brush.

You’ll make more mistakes when you try to cut in if you don’t have a good brush (plan to spend $15 to $20 for high-quality, angled brush) or if you don’t have enough paint on the brush.
 
Don’t just dampen the brush with paint, saturate it. Wipe off one side and paint your straight line. Still, you don’t want to get your arm into the paint up to your elbows.
 
A rule of thumb: Cover all but the top half-inch of the bristles with a good dose of paint.

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