Solve problems from Codeforces with difficulty ratings between 900 and 1500 that are tagged with bitmasks. For each problem, visualize the logic using bit-level representations. Focus on helping understand how bit manipulation or bitmasking is used in the solution.

I would suggest you to watch some videos on bitwise operators and learning about a few identities. Also, learn how to manipulate kth bit of a number. Also, when faced with certain bitmasks (at unrated level), try this thing, first try to solve it for a binary string instead of an actual array of numbers. Then just repeat the solution for 32 (or 64) binary strings combining to form the numbers.