Figuring out self-employment tax installments for the first time

(Asking on behalf of someone else)

 

I have never before been self-employed and am trying to figure out tax installments. I provide childcare in my home for a family member and they pay me. (If it matters, they'd like these payments to count toward their dependent care FSA.) 

 

I am married with a child of my own and we file jointly. I do not expect this income to continue for the rest of the year and I expect to earn less than $10k total. My spouse has the primary income in our home. 

 

How do I estimate a quarterly installment given that my expected earnings are so low, this income likely won't last the whole year,  and I file jointly with my spouse? The number of variables makes my head spin. Thanks!