I put the mat down and waited for her to get in it. Then I c&t, then wait, then release. I slowly increase the amount of time she stays in before I release. To get her up on the release cue I had to act very excited at first. Once she was going there often, I would say "park it" as she got close, c&t, release. When we got to longer lengths I replaced C&t with several treats over the time span. Practice LOTS with short reps before making it long.
If she gets off before the release word, have her go back, wait, release with no treat.
If she gets off before the release word, have her go back, wait, release with no treat.