adding data to wp tables

I’m writing a plugin and i’ve set up it’s tables in the wp database. They show up in phpmyadmin and look fine. However, I cannot figure out how to add data to the tables from a form. I’ve utilized the creating plugins codex to no avail. No matter what I try, I cannot seem to get data in the tables. Am I doing something wrong? Can someone post a correct method to do this or point me in the right direction?