Start by downloading the software from the TeamViewer website:

The file should be in your downloads folder called Double-click to open it. 

It should unzip the TeamViewer program that is needed.

Drag the icon in the window to your “Applications” folder to copy it to your computer. Then double-click to start it.

Click the Open button on the security prompt.

Allow TeamViewer permissions to access your computer in the next window. Sequentially click on each button and allow it access.

Clicking on "Request Access..." or “Open Screen Recording Preferences…” will open a small popup. Click on "Open System Preferences"  and the “Security & Privacy” preferences pane will open. 

Check the box next to TeamViewer. The preference window will warn you that you will need to quit TeamViewer, click “Later”. Do not close the preferences window yet.

In the same preferences, window click “Accessibility” and then unlock the preferences pane by clicking the Lock icon at the bottom left of the window. You will need to enter your username and password for the computer. Once unlocked, scroll through the right panel to find the “TeamViewer QuickSupport” item and check the box. Do not close the preferences pane yet.  Note: If you are seeing the entry TeamViewer_Desktop in your list, make sure to check this box as well.

In the left pane of the preferences, scroll down to “Full Disk Access” and select it. On the right pane click the “+” icon at the bottom of the list.

In the file chooser window,  click the “Application” shortcut and then locate “TeamViewer QuickSupport” and select it from the list. Click the “Open” button. A warning dialog will appear, dismiss it by clicking “Later”.

Click again on the padlock in the lower-left corner to prevent further changes and you can close the Security & Privacy window. You can now click the “Close” button on the TeamViewer “Review System Access” dialog.

The application will then present you with the credentials to provide to your technician.

The next time you want to receive an incoming connection with TeamViewer, the person connecting to your Mac will be able to see and control your device.

The permission status can be reviewed under Help -> Check system access at any time the app is opened.