Don't reach up. Please. I'm begging you. Unless you want to give your arm away like a daughter on her wedding day, don't reach up. If you MUST submit someone from the mount....
1) Put your hands in the folds of his hips
2) bump hard with your hips, push with your arms at the same time (not pushing straight up, more at a diagonal away), and scoot your butt back after you finish your hard bridge (there should be space now, if not, repeat step 2 on opposite side, continue until space is made)
3) bring one knee inbetween his legs (ala butterfly guard)
4) Wrap your other leg around his leg, and put your heel in his hip/rib with your toes facing away from his ding dong (DO NOT PASS THE HIP, REPEAT, DO NOT PASS THE HIP WITH YOUR FOOT!)
5) push with the foot on his hip, and your knee
6) Overhook his ankle
7) Ankle lock
Note: At step 3, you have the option to just go to butterfly, or X-guard depending on their reaction. Once again to emphasis: DO NOT PASS HIP, DO NOT COLLECT 200$. That is considered reaping the knee, will get you DQ'ed at tournaments, and will cause a lot of butthurt against your less leg lock aware training partners.