Write out the following numbers: 1, 2, 4, 8, 16, 32, 64, 128. Each number is double the previous number. Learn the sequence all the way up to 1024.
Write the sequence out backwards on a piece of paper, beginning with 1024. Leave space underneath to write binary.
Pick a number between 1 and 1024. To make this number, you work out which of the sequence add up to that number. For example, you can make 100 by adding 4, 32 and 64. Place a 1 underneath these numbers. Place a 0 underneath all the other numbers. Starting with the first 1, you get the sequence 1100100. This is 100 in binary. Test it again using other numbers.
Practice doing binary without the sequence written out. You will soon be able to work out numbers less than 100 in your head. Once you get to higher numbers, it will become harder, and you may need to write it down on a piece of paper. However, once you know the basic technique, you will be able to make any number.