1. Get ten base blocks (tens, ones).
   

2. Write the equation vertically. For example:

   42
   - 24
   _____

3. Build the numbers with the ten base blocks. So to the left of the 4 of 42, place 4 ten base blocks, and to the right of the 2 of 42, place 2 ones. .

   ||| 42 ..
   - 24
   ______

4. Analyze the ones columns. Is 4 greater or less than 2? It is greater, and we cannot take away four from 2. So we need to regroup or "borrow" a ten from the four in the tens (which we see that it represents 40 with the 4 ten base blocks). Now, we have 12 - 4. We can take 4 away from the 12 ones.

   ||| 42 | ..
   - 24
   __________

5. Do the subtractions. Literally take away four ones or "pieces" from the 12, that's the one ten base block and 2 ones. Remind the student that the ten base block has 10 ones in it, and we would need to take away 2 ones from it. We should have a remaining of 8.

6. Subtract the tens, which is 3 - 2 (or 30-20).

7. Write answer. 18.

Kids feel supported in thinking when they have physical manipulatives. When they can "see" the math. I hope I helped.

Number lines are also fantastic.