There are two ways to generate invoices for memberships - automatically from a membership page and manually after a membership transaction with the Pay Later option. 

Automatic Invoices for Memberships

The automatic invoicing option is located on the applicable membership page. Create a new page or edit an existing page. 

Proceed to the Membership Settings Tab. 

Under the Financial Information Section, switch Allow Purchaser to Pay Later to "YES." 

Below that switch Automatically Create Invoice if Pay Later to "YES."

Make your other applicable choices and click Save & Next to proceed. Once all the sections and tab are complete and the page is active, an invoice will be generated along with each Pay Later transaction. 

Manual Invoices for Memberships

If the automatic invoice option is not turned on and a membership transaction is processed as "Pay Later," the membership status will be Pending.  To create an invoice for a membership with an outstanding payment due, you will create a new invoices from the main Invoices Grid in the Transactions & Invoices module, click on +New then select Invoice, or from the constituent's Invoices Tab, click on Add, then Invoice. Check the box next to the membership and add-ons from the Outstanding Balances section and complete the rest of the fields. Click Save to complete the invoice, Save and New to complete and start another, or Save and Print to complete and open the Print/Email Mail Merge window.

Note that if the memberships are not listed in the Outstanding Balances section, there may already be an invoice created or the membership has already been paid.

For more information see the article "How do I create an invoice manually?"