How can I convert just some of my accounts to see how permission sets work?

You can convert individual accounts by selecting Permission sets / Allocate from the Resources menu. Click on the permission set you want to allocate, and then either enter the username of the account you want to convert in the Allocate permission set to single account field, or else click on the Allocate/remove permission set to multiple accounts link to convert a set of accounts.

If you have sub-administrators, you can impersonate one of them and then use the Account Convert Tool to convert all the user accounts beneath that sub-administrator. You can also create a sub-administrator account for this purpose, even if you then create just one personal account beneath it, to see how the Account Convert Tool works before using it on all your accounts.

You can also use bulk uploads to convert existing accounts to use permission sets. You might want to do this if you use bulk uploads to refresh Athens accounts on a rolling cycle.