At the at end of the apron, traverse right (south) to get back inbounds. If you ski too far left you'll get caught on flat land and have to hike out.
You can access the Thumb either from the Bridger Ridge hike or touring up The Ramp
ascent line. I found that the boot pack from Bridger is my preferred route. Make sure you hike past Northwest Passage and through the OB gate. Keep hiking down until you find the thumb rock (the unmistakable rock pillar that looks like a thumb). Head north past the rock.
The first entrance is steep and technical. The second entrance is farther north and is a bit easier, but you'll need to traverse back south to the Thumb's entrance. Once inside, there is a rocky cliff in the center that you can ski around to the south or huck it. After that, it's all yours.