Here are the steps...
1) Create a new Webmin user, and assign which modules the user is allowed to access (that's done on the same page where you choose username/password, etc. for the new user)
2) After the user is created, go back to the user list where you'll see the user listed with the modules that you selected.
3) Click on any of the module names for that user -- Apache Webserver, for example.
4) At this point, the options are based on the module you choose. For Apache, you can select things such as which virtual servers the user can edit. For the BIND module, you can select which domains the user can edit.
That's basically all there is to it...