Here's an up close pic of where I taped off in our kitchen. I hadn't decided what color to do in the kitchen, so I stopped along the corner. I had some difficulties in deciding where to stop because I have rounded corners, but I decided to stop right before the corner. I don't think it really matters as long as you stop consistently on all the other corners.
1. Tape off the area you want to paint.
2. Apply a thin layer of matte medium along the edge you want to be crisp.
3. Allow to dry.
4. Apply a layer of paint. Allow to dry. (apply another layer of paint if necessary)
5. Once paint has been allowed to dry (I waited at least 6 hours), remove the tape.
Just make sure that you don't apply the matte medium or paint to thick. Also, just use a back and forth motion along the edge of the tape.