For a student, what is the optimal usage of a solution manual?
Never use the solution manual before trying everything else; talk to friends, visit the professor, go to class and listen (!), check the internet. Once you use the solution manual for a problem, the potential gain from that problem is significantly and irrecoverably reduced.
Use the solution manual to check your work. (Duh.)
For problems you aren't planning on solving, you can use the solutions manual to create flashcards and other learning aides (if the course material is anemable to such a construct).
If you have a friend/roommate/spouse/trained monkey who can compare your answers to the manual for you, such that you don't actually read through the manual, that may be useful for certain topics.
You can make some good money selling it when the semester is over :)
Ignore it and write a new one.
Looking at the solution manual is not useful; it only gives you answers. The point of homework isn't the answers, but the struggle to find them.
Don't just read the solutions. This is counterproductive. Work the problems yourself.
However, after finishing, use the answer (not the worked solution) as a check. It is psychologically reinforcing to have a feedback loop and know that you did it right.
If you were wrong, rework the problem. Often the reason will have been a minor calculational mistake (in STEM). You will usually be able to fix this on your own (without even seeing the detailed solution) just by being more careful. If not, sometimes the format of the answer will suggest a more conceptual thing that you missed or prompt you to relook at the textbook description/examples. This is crucial, that you fully redo even "dumb mistake" attempts. Doing this means you won't make them again. It's like music or sports. If you make a mistake, do it over. Do the entire exercise, not just the part you messed up.
In the few cases where the above is not sufficient, look at the actual worked solution to see how they do it. But STILL. Then put the manual down and rework the missed problem yourself. You need to actually practice the solution process. Not just read it. Even though it seems hokey, it will help your learning, versus reading and saying "OK, that was the trick".
I do disagree with other answers that say you should prioritize outreach to friends, Chegg, instructors, SE, before checking the solution manual. It is a tool for you and is extremely convenient for the disciplined drilling problem solver. I would reserve that sort of personalized outreach for when you are baffled by the written solution itself. (Keep a written list and then see your instructor with them.)
But absolutely do use the disciplined drill, check, repeat (if wrong) method that I espouse above.