First design your subform as a form in it's own right.
( In the design phase there is no difference between a form and a subform - indeed I have databases where a form can appear in it's own right as a form - or can be used as a subform within another form. )
If this is intended to be a permanent subform, however, I tend to call is subfrmXxxxXxxxx to differentiate it from all the main forms that start frmXxxxxXxxxx . That way, when you are looking for a subform in a list of 100 forms then they are easier to find.
Save the new sub form.
Create ( or Open - if it exists already ) the Main Form
Select the SubForm/Report control from the toolbox.
Allow the Wizard to do most of the hard work and select the newly created subform when the wizard prompts you.
Fine tinkering may be necessary - depending on what you're doing - but you will, by now, have a subform on your main form.
QED.
G LS