Every Skill has a switch in its banner that lets you mark the Skill as "ready".

Turn the switch on once you've done any necessary preparation (such as reviewing the Directions or References) and all you need is the opportunity to meet with a Mentor for final sign-off.
If you turn the switch on, then it looks like this:
...and the Skill's Point icon will also be the bookmarked: ready_points.gif
Once a Skill is Ready, you'll see a Ready option in the side menu:
This option will show a list of just your Ready Skills. You can use this list to keep track of the Skills you're ready to perform.
Since you just need a Mentor in order to complete these Ready Skills, Pract.us will automatically notify you whenever any of the Mentors in any of your Ready Skills become available. You don't even need to follow them. You can choose the way you want to be notified (or even turn it off) with this option in your Profile > Notifications
Note that if you ask a Mentor to sign off a Ready Skill, and the Mentor rejects the request (this usually happens via the email sign-off method) then the Ready switch is automatically turned off. Basically, the Mentor believes you're not quite as ready as you should be!