Open the Find My Device website. Go to https://www.google.com/android/find in your browser.

Log into the website. Enter the email address and password for the Android that you want to lock.

Select your phone. On the left side of the page, click the icon for the phone that you want to lock down.

Click LOCK. It's a tab on the left side of the page. Doing so opens a menu below the LOCK button.

Enter a password. If your Android doesn't have a lock screen passcode, you'll need to type a temporary password into the "New password" and "Confirm password" text fields.

Enter a message. Type a message to display on the lock screen into the "Recovery message" text field. This is optional, but recommended if you suspect that your phone was misplaced rather than outright stolen.

Add a recovery phone number. Type a phone number at which you can be reached into the "Phone number" text field. This phone number will be displayed on your Android's lock screen.
As with the recovery message, this step is optional.
Click LOCK. It's a green button at the bottom of the page. Doing so will place your Android under a lock, making it nearly impossible for someone to break into it or access your data.

Erase your Android if necessary. In a worst-case scenario, deleting your phone's data entirely is better than losing it to an unknown thief. To erase your Android phone, select it, click the ERASE tab, and follow the on-screen instructions.