Pay retroactively?

Hi there!

 

My income is inconsistent, how do I determine how much to pay? I’ve made less this year than last year and I file Jointly with my husband.  I was supposed to pay $1900 quarterly, but I didn’t even make that much for the first or second quarter. 

 

I haven’t paid any yet, should I pay retroactively? If so, how do I determine how much? Do I include the stubs the IRS provided? 

 

Thanks!