I've worked in year 6 for years. I love year 6 maths!! You should start at the left and work to the right. It's always like that in maths. Always work out each column at a time.
The 'ones' column on the left shows the sum
7 - 5 = 2
The next column across is the 'tens' column and has the sum
? - 3 = 9
We know that 12 - 3 = 9 (because we know that 9 + 3 = 12), so 12 goes above the 3 ... except that you can't put two digits in one box. You have to think of the 12 as 10 + 2. So you put 2 in the box above the 3 and the 10 becomes a 1 in the next column along to the left. (That column is the 'hundreds' column and is therefore worth ten times more than the 'tens' column, so the 10 doesn't need it's 0 when you move it along to the next column) (I can explain that more if you need me to!!)
So, you need to add that 1 to the 4 in the 'hundreds' column. 4 + 1 = 5.
Moving left again into the 'thousands' column and you've got a sum that says
? - 2 = 8
We know that 10 - 2 = 8 (because we know that 2 + 8 = 10)
Again, you can't put a two digit number in one box, so you split the 10 into a 10 and a zero. Put the zero into the box above the 2 and add the 10 as a 1 into the next column along.
Now you need to deal with the final column, the 'ten thousands' column. You've got a sum there that says
? - 4 = 2
We know that's 6 - 4 = 2. But you now have to add the 1 that you brought over from the previous column to the 6, so the answer in that column is now 7.
Done!
It's all about remembering to deal with each column on its own before moving onto the next column. Always start on the left or it won't work. That's the same if you're adding, subtracting, multiplying or dividing.
Hope that helps! It sounds complicated but as long as you follow all the steps in the right order it's easy enough!!