SE quarterly taxes for single owner LLC

I have a counseling practice, which is a single-owner LLC and one contract employee.  I am the therapist who sees all the clients and the contract employee does office work such as billing.  We do not have a payroll system set up, we just manually write paychecks for the two of us weekly with no taxes withheld.  The contract employee makes a fixed amount and my payment varies from week to week.  How do I calculate what the quarterly taxes withheld should be, and how do I catch up since we have not paid quarterly taxes this year?  We use quickbooks for our bookkeeping.  Would it be easier to set up payroll for the two of us, but does that have to be a fixed amount every week? Thank you!