For example, you want to 100 decimal to a binary number in zeroes and ones. You keep divide 100 by 2 until you have 0 left. And the remainders will be a binary number. 1100100. The first 1 is the remainder you get by dividing 1 by 2. The last 0 is the remainder you get when you first divide 100 by 2.
The easiest-to-understand way of converting from decimal to binary is, first find the largest power of 2 that is smaller than 100. And substract that power of 2 from 100. And find the largest power of two that is smaller than that difference we found. Repeat this over and over until you get to 1, which is the two to the zero power.