Day 0: Put the tracking code into your HTML

1. Get a few lines of tracking code from the Matomo Admin panel
   Dashboard >> ⚙️ >> Measurables >> Tracking code  OR Dashboard >> Tag manager >> Install code "< >"
2. Paste that code into every page of your site. The easiest way is to paste it just once, into your header. Or footer.  Caveat: do you have any pages that don't use your header or footer? They will be unmeasured unless you insert the Javascript tracking code on those pages too.

See examples of tracking code in the HTML source code of these:
https://pikl.us
https://littlefurnace.com
https://tatll.me
https://genest.weebly.com

The last link above is using a tracking pixel.