How do I paint stripes on a wall correctly?
I hate to tell you this, but you will have to either start over or make the stripes wider when you redo them. Unless you are some "gifted" artist, you aren't going to be able to free-hand those lines.

The hardest part about your dilemma is the fact that the wall has a rough surface. It's going to be pretty hard to achieve a "perfectly" straight line. However, the smudges can be prevented.

First of all, you can't use just any painter's tape. I used 3M Safe-Release tape for years until I found a few better tapes. Frog Tape works very well and they have different types depending on your surface. I recommend using Frog Tape Multi-Surface for your project.



Taping is not the only thing to keep in mind. When you paint your stripe you need to do what is called "feathering". Dip your brush and then wipe it nearly dry on the edge of the paint can. Apply the paint "away" from the edge of the tape. If you paint toward the tape edge you increase the chances of pushing the paint underneath the tape. Fill in the stripe by feathering it as well. The stripe will not cover the paint underneath yet. It should appear streaky and light. This is intentional, as you are creating a "tack coat" for the finish coat to better adhere to.

After you feather the stripe, wait about 30 minutes for the paint to dry about halfway (for latex paint). Afterwards, go back and feather the edges once more and fill in the stripe again. The stripe will be much richer and will cover better. Sometimes you might need to do a third coat. If you know that the color of your stripe isn't going to cover well (e.g., Pink over Yellow), you might need to use a stain blocker on the first coat. However, you will need to apply it very lightly and allow to completely dry before applying any finish paint.

I like to remove the tape while the paint is still wet. It prevents issues with the paint possibly sticking to the tape.