Hi Laurie,

For this kind of thing, you could use masking tape. The line will be just as straight as your tape is. When you do it with 2 persons (one putting the top in its place, and the other below), you can put on a long strip of tape in one time. Then you'll get the straightest line. The tape should be locked to the wall all-over (on the seam edge), otherwise the paint will creep under the tape. You sort of dab the brush on the tape, not wipe the paint under it.

Also important: remove the tape ASAP, otherwise it will get stuck too good, and take paint chips or more with it when you remove it.

Good luck with your job, Evelien