The Skill tests are a part of the Guided lessons you play and you can also find them via the Learn → Missions tab.
Once you complete a Skill test, select Continue and Make my certificate. The information is pre-filled for you but you can also change it on the certificate screen. Select Make my certificate, to have the certificate emailed to you.
I can't find my certificate
If you passed a Skill test but can't find the certificate in your inbox, check your spam/junk folder, as the email could have landed there. If you want to find a certificate for a previous level, search for "Yousician Certificate" in your email. You can find the certificate included in the email and you can access it at any time.
If you have lost the email, you can re-play the Skill test and re-send it to yourself. Or, you can reach out to Yousician Support, who can email you a copy. Note that Support can not change the first or last name on the certificate.