Maybe I'm stating the obvious, but one thing to do is to find problems that are similar to the type you want to write and use them as models. Then use the pointers Gavin gave to do specific things.
When you do this, you should look for problems that are written using current techniques, i.e. ones that use Davide Cervvone's MathObjects. Many WeBWorK problems were written many years ago. They still work but using MathObjects gives you (and the students) many advantages.
You can recognize such problems by looking under "loadMacros" where you will see things like PGstandard.pl, MathObjects.pl and/or Context*.pl. Also in the body of the problem you may see constructions like Context()-> and ANS($something->cmp).
In the Library Browser click on "NPL Directory". I believe all the problems you will find under "Union", "Michigan" and "CollegeOfIdaho" use MathObjects. You can click "Edit it" to see and copy them. The other collections will contain a mixture of problems using old and new techniques.
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