1st Step: Create the Smartlist with all customers.

Marketing > Smartlist > Add a list > Name: "All Members"

Do not add a filter because by default this Smartlist includes all clients.

2nd Step: Create the mail / newsletter.

Again, in the Marketing > Emails tab, create a template or use one that has already been built.

Choose the name of the template and the subject of the email.

Add blocks, images, text, buttons etc... and save.

3rd Step: Send the email.

From the Marketing > Smartlists tab, select the "All Members" Smartlist. Click on send email, then select the email to send. Click on "Send".