Multiply 1 by 2 and make a note of the answer (2).
Multiply that by 2 giving 4 and keep going until you have as many numbers as digits in the binary number.
If you had to work out 10010101, you would need repeat the above step eight times.
List the all the answers you get, starting with the highest number on the left and lowest on the right then underneath, write the binary number so
128, 64, 32, 16, 8, 4, 2, 1.
1.......0....0....1...0..1..0..1
Add the numbers together that have a 1 below them and that will give you the number so the above example is:
128+16+4+1 =
149
I hope that makes a bit of sense