First, I want to concur with my colleague Eric:
If you are not attending Mass on Sundays, you are committing a mortal sin each time. Our Lady would want you to prioritize going to Mass on Sundays.
Whether you are practicing First Friday devotions or First Saturday devotions (or both), you can go to any valid Catholic parish to receive the sacramental aspects of the devotion. I.e.:
- Going to Confession, in a church or rectory or
- Going to Mass and it doesn't have to be the same church or church time.
I currently go to my First Friday Mass at my local parish, St. Patrick. Because a morning Mass is not available at my parish on First Saturday, I usually get up a little earlier and drive to St. John the Evangelist in Hopkinton for their First Saturday Mass.
I have been practicing both First Friday and First Saturday devotions for years.
To ensure I receive the sacraments in a state of grace, I always mark the day before First Friday, as Confession Thursday. This ensures that I always receive the Blessed Eucharist in a state of grace for these devotional days as well as the following Sunday morning when I go to renew my covenant. (meaning go to Mass).
As far as the other aspects of making your First Saturday:
- the recitation on the five decades of the Rosary can be done anywhere: in a church, a car, in an adoration chapel, where ever,
- the same is true for the 15 minutes of meditation on all the mysteries of the Rosary.
I have a segment in this posting that is related to these issues:
I hope this helps,