I keep track in a spreadsheet. When I make money I enter how much I earned and how much of it should go to Maaser, I usually round up a bit to make it a little more than 10%. I then add all donations I give into the spreadsheet and it calculates how much I owe, how much I gave, and how much I have left to give. I can then look back next year to see how much I gave to each cause, etc.
I try to make it work with taxes, if the take taxes off a paycheck it’s easy because I juts have to enter how much I actually got. If I get a refund I will count that as money made, and if I have to actually pay taxes I will enter it as negative income so taht I will then owe less in maaser