You can create custom fields in one of two ways:
- The simple way - by configuration. No coding required. Choose this if you're not a .NET developer, or if you just don't need any custom configuration and validation of your field (mandatory validation is supported out of the box).
- The advanced way - by coding. Choose this if the statement above doesn't suit you :)
No matter which of the above you choose, you'll need a field icon. It has to be a 16x16 px PNG and it should be grayscale.
If you want your field icon to have the same look and feel as the ones shipped with Form Editor, head on over to http://p.yusukekamiyamane.com/, pick an icon from the Fugue Icons set and make it grayscale.
The samples section contains a couple of custom field samples. Check out creating a simple custom field or creating an advanced custom field for a run-through of these samples.