Try to work through sections in order, because each one builds on ideas from earlier sections. Graph problems often use DP. Constructive problems may need number theory insights.
For each problem, open it on Codeforces and try solving it yourself first. Spend at least minutes before reading hints. If you get stuck, read the first hint unit, then try again. Reading solutions without struggling teaches you nothing.
Submit your solutions on Codeforces to verify correctness. The judge there tests edge cases that you might miss when only thinking through examples.