If you need to process a membership internal on behalf of a constituent, there are two ways to go about it. The first option would be to launch your current Membership form and populate all the fields based on that Constituent’s request.
Another option if the Constituent already exists within your database would be to go into their Constituent record and onto their Membership tab. Here you will see a green “Add Membership” button. Clicking this button will allow you to select which Membership form you wish to launch, and it will pre-populate the billing information on the form for the Constituent whose record you launched the form from. It will even allow you to purchase the Membership using a saved credit card on that constituent’s record.