Custom iParts, how to make and use them

Create your part normally, for example a cylinder with Diameter and Length dimensions.

Clicking on the Create iPart icon you'll get the iPart Author dialog, right click on the Diameter column and select Custom Parameter Column:

The diameter column will become purple. Do this with all the other parameters to make the custom iPart:

When you place the custom iPart inside an assembly you'll be shown this dialog, where you can change the values:

Here is an example of the 3 custom iPart cylinder instances inside an assembly:

How to change parameters of an already created custom iPart.