Arch Linux: Adding a self-signed certificate to trusted

To add a self-signed certificate to trusted, run the command:
sudo trust anchor --store my-cert.pem

View all certificates in the trusted list:
trust list

To quickly find the required certificate, redirect the output to the "less" utility and use the search inside it via "/". 

Removing a certificate from the list:
sudo trust anchor --remove "pkcs11:id=%AA%BB%CC%...DD%EE%FF;type=cert"

where "pkcs11:id=%AA%BB%CC%...DD%EE%FF;type=cert" is the URI of your certificate (looked up using the "trust list"). You can also use the file name of your certificate instead of the URI, as when adding.