If you already have the Internal Release Form customized the way you want, enabling it is very simple. In Admin > Tickets > Internal Forms, you'll see the Release Form listed at the bottom of the grid. This should the place where you customized the form.
You'll see that it says No under the Enabled column. Simply click on No and it will enable the Internal Release Form.
Once you have that enabled, all internally created Release tickets will use that form.
As for tying custom forms to types, that's a bit of a different story. It is possible to do, but you will have to tie the type to a service in the service catalog and have the tickets created through there. First, you'll need to customize a Request Form to
look how you want it to, which you can do in Admin > Tickets > Request Forms. Once you have your form configured, you can then tie it to a service. Navigate to Admin > Service Catalog > Services, select the service you want (or create a new one). You'll need
to make sure the Request Type is set to Ticket, and then you'll use the Request Form filed to select the form you just customized. At this point, you can also tie that service to a ticket type using the lookup directly below the Request Type field.