What software are you using to prepare the return? Usually you can enter banking information for electronic withdrawal in the program. If it asks for prior year AGI, you can enter zero per the IRS website.
A lot of people still pay by check although the processing time is longer, and it makes me nervous given the backlogs and confusion at the IRS.
I have someone do my taxes and I always pay on irs.gov for myself. This is the 1st time that a minor owes taxes.
I am pretty sure that when you go to enter the payment, though, that previous poster is still correct and can just enter $0 for the prior year as part of the identification.