Imagine this simple custom list created in Sharepoint 2010 :

If you want to add an item to this list, here is the dialog that will be shown:

To easily customize this form:

Go in the List Tools > List in the Ribbon:

You can then choose the « Customize Form » icon :

Or, if you go in the Library Settings, you can use the "Form Settings" link.

Infopath will open with the form you asked to customize.

Here is my customisation (I will probably isn't it ?)

Now, when you create or edit an item in the list, here is the form that will be shown :

And if you browse your list in Sharepoint Designer 2010, you will find the template.xsn in the Item folder of your list.