Add Membership Widget or Buy Button to a page
First, go to the Existing Plans page. Here, you will find your newly created membership plan. The Existing Plans page is where you can view and manage all your plans.
Click the gear icon under the Actions column. You will want to decide if you want to use the Buy Button or the Widget. You don't need to use both on a page.
The Widget will show the sign up form on a page, the customer will fill it out and hit the button to buy the membership.
The Buy Button will display a button on the page. So say for instance you wanted to have the page less cluttered and not show the form you would select that option. Once clicked it will display the form in a modal window, after filling it out the customer can complete the purchase.
Once you've figured out which one to use, you can go ahead and grab the code for your choice.
With the code copied you will then paste it into the page you want it displayed on. Generally this is done on a specific page you create for the Membership, not on a product page.
Save the page, and go check it out!
Enable customer accounts
You will need to go into your Shopify Admin, Settings -> Checkout. Set accounts to Optional.