。
Use your free time as much as you can.
Literal
Free-time [object-を] as-much-as-possible use-[soft-imperative-なさい].
できるだけ ('as much as possible') is a fixed adverbial built from the potential できる plus the limit-marker だけ — literally 'the extent one can.' It attaches to the following verb and is one of the most useful set phrases for giving advice in Japanese.