If you mean a name or some such, yes, I put on a name a year or so after getting the trailer.
I put on a straight line using masking tape. Besides keeping the decal straight you can mark the center and from it the start point for placing the decal.
Do make sure the area is very clean, any dirt will show up when you squeegee. Use lots of soapy water if that's what you'll be using. Don't do it in high heat or the sun. It's been a while but I seem to recall you have to pay attention when peeling, some of the leading edges of character didn't want to separate easily.