Flip-flops can have a value of 1 or 0, a.k.a. binary, or base 2. To change from base 10 (our normal counting base) to base 2 (or 3 or 4 or 8 or 16, or any other base) by hand, you will need to do some division (r stands for remainder):
So to convert 11 to base 2:
11 / 2 = 5 r 1
then 5 / 2 = 2 r 1
and 2 / 2 = 1 r 0
finally 1 / 2 = 0 r 1
Next, gather the remainders in reverse order: 1, 0, 1, 1. This will give you 11 in base 2. As a proof, convert 1101 back to decimal: