This page will show you how to use Skills in Seesaw. In your selected class, you'll see the right-most tab called 'Skills', so click on that. If you have never before created a skill, you won't see anything in the window, so to start, you will need to create a skill. Click on the add 'Skills' button towards the bottom. Fill in all the fields, and then submit.
Once a skill is created, it will show you a chart with student's names and in the y axis and the skills in the x axis. To add points for students, you can take a look at their posts and in the bar below you can see a graduation cap icon (same place where the like and comment buttons are). If you click it, you can choose which skill to "tag", and then it will change the number in that skill's column next to the student's name.