First you take a piece of paper and you write the number 783. Directly below it, and in line with the numbers above it, you write the numbers 366. You will start at the right and work your way left. First you will look at the three. Since three is less than the six below it, you will take a "tenth" from the number to the left of the three. The 8 becomes a 7, and the 3 becomes 13. 13-6 = 7.
So, the bottom right number is a 7. Move one space to the left. Now you see 7 minus 6. That value is 1. Now working from right to left your answer so far is 7 and then 1.
Finally, you see seven minus 8, which is 4. Which brings your total to 417.