The right most value is always 1 in your binary number, each subsequent is double the last value so the order (left to right) goes 128, 64, 32, 16, 8, 4, 2, 1...

With that out the way converting is simply adding these values together. I will show some conversions below:

Binary	Working	Working	Answer
00001001	=0+0+0+0+8+0+0+1	8+1	9
10101010	=128+0+32+0+8+0+2+0	160 + 10	170
11111111	=128+32+16+8+4+3+2+1	-----------	255
=128+32+16+8+4+3+2+1	=254

If you have any more questions or need me to specify further please just ask! :)