There are two things to customize in signup form:
- design / look managed by templates
- contents - what fields should be displayed
Changing the affiliate fields
In the default installation the affilaite has fields like Username (Email), First Name, Last Name, Web Url, Company name, Street etc. You can change some of these fields, or add another fields that will gather another data.PAP allows you to define up to 25 fields that can be filled with additional affiliate information.
To edit affiliate fields, open merchant panel, go to Configuration -> Affiliate signup to tab Fields.

The first 5 fields are fixed, and cannot be changed. The next 25 fields named data1..data25 can hold additional affiliate information.
You can see from the screenshot that for every field you can set if it should be mandatory (must be filled), optional or disabled (not displayed). You can also choose field type type as Textbox, Number, Checkbox, Listbox or List of countries.
Changing the signup form design
In the tab Customize design you can see links that will open template editation for the most frequent sections in the signup form.

Now click on the Edit signup form link to open template for signup form main contents.

This will open the signup form template in our standard template editor . There you can edit the template file, and you also see the location of the template file, so you can edit it manually.
The constants like {widget id="lastname"} or {widget id="data1"} are our Smarty template engine tags that tell the system to insert field with lastname or field with data1 to the form.