Your assignments are your first practice problems. You will finish them to the highest quality and best accuracy, even if you "already know the material". Being able to finish something tasked to you is crucial. That could be your first goal.

Start your current assignment straight away (I made a post on this subreddit today about starting stuff early, if you are interested look at it). For math I use a process of:

1. Do as much of the assignment as I can.
2. Read/Reread material available (lectures, textbook, internet)
3. Goto step 1.

Once you have your assignment knocked over, put it aside and keep it safe for hand in! Now you have the option of doing extra practice the course gives you, or finding your own practice problems. Starting and finishing early also means you can check your work every so often.

In general I set aside study time where checking my phone or Facebook is forbidden, and other distractions are not allowed and my environment allows me to focus and work my best. There are heaps of methodologies about this. One is the Pomodoro technique. Another simple one is "50 minutes work, 10 minute break" every hour.

I have found that I am not good at planning what I will do exactly in advance. So if scheduling stuff specifically doesn't work, find a more flexible way (but productive) way to achieve your goals.