Let's say I'm creating a shop that sells and prints basketball jerseys and I want the customers to be able to enter the name and number that they want on the back of the jersey when they buy them. How do I accomplish this in Magento?
I believe Magento has a built in feature to help out in these situations!
Go into the backend. Catalog > Manage Products > Product you wish to add this on.
Once on this at the bottom left there will be a tab that says 'Custom Options' it is in here that you set up this feature. All you do here is click 'add new option' (x2) Title the first 'Name', and the Second 'Number' and set them both to 'Text Field'. And ensure the sort order is set correctly so name is set first.
The rest you configure as you wish.