Step 1: Know who gets it
In the admin menu: Marketing, then Email Marketing
An announcement goes to every client with an email address who has not unsubscribed; the screen calls them "Everyone who has opted in". Clients who sign up on your website can be emailed from then on, until they unsubscribe from a link in one of your emails; clients you add yourself, or who arrive through a registration, cannot be emailed until they opt in. On a paid plan the "Contacts" tab shows who can be emailed and lets you switch someone off, and you can pick an audience instead of everyone. The page says how many people that is, and how many addresses are suppressed because they bounced or reported spam.
Every email carries an unsubscribe link, and anyone who uses it is not emailed again.
Step 2: Write it
In the admin menu: Marketing, then Email Marketing
Under "Send an announcement", fill in the "Subject", an optional "Preview line" (what inboxes show after the subject) and the "Message". Add a "Picture (optional)" and describe it in "Picture description" for people who cannot see it.
Put your own address in "Send yourself a test" and click "Send test"; the copy you get starts with [TEST] and nobody else receives it. "Preview" shows it on screen.
Step 3: Send it
In the admin menu: Marketing, then Email Marketing
Click "Review and send". The dialog says how many people will get it; its button names the number. Once it starts it cannot be unsent. On the free plan it then appears in the list below the form; on a paid plan, on the "Campaigns" tab.
On the free plan there is a monthly limit on how many emails you send. A note appears beside the form as you near it, and at the limit you cannot send until the next month. A paid plan adds campaigns, audiences, scheduling and open and click tracking.
Read Run an email campaign