A portal to Cathedral will open automatically once you've beaten Mom's Heart 11 times.
Simply finish Catherdral by defeating
5 times.
You can use the same character 5 times if you wish, but using 5 different characters will unlock 5 achievements alongside it (With the exception of blue baby...this will unlock the D6 for use for Isaac)
Alternatively, you can reach Cathedral after the Mom's Heart fight by having an Angel deal precedent, but this isn't always reliable.