How do I get a custom record type to show up on a custom subtab? I have a manufacturing guide that employees can fill out that should be referenced on a sales order.

Is the Sales order the parent record?

Yes

On your custom record, you have a field of type List/Record -> Sales Order. In that field, tick

RECORD IS PARENT

and then under the Display tab select the

PARENT SUBTAB

where you wish the custom records to appear.