Assuming that you want to sort from smallest to largest:
Create two groups: one that is sorted, one that isn't.
Before you start, there are no items in the sorted part, all are in the unsorted part.
As you work, place the sorted numbers before the unsorted numbers.
Go through the unsorted list and find the smallest number. It is 20. Place this number in the sorted part (at the beginning) and leave the rest alone:
20, 70, 40, 90, 100
Keep doing this, always adding to the sorted part and reducing the unsorted part.
The second time through, go through the unsorted part (70, 40, 90, 100) and find the smallest number in this sublist and place it at the end of the sorted part.
You now have: 20, 40, 70, 90, 100 (the sorted part is 20, 40 and the unsorted part is 70, 90, 100 -- OK, it's sorted but you don't know that yet)
Do it again, starting with 70: (the smallest number in the unsorted part is 70)
You get 20, 40, 70, 90, 100 (the sorted part is 20, 40, 70; the unsorted part is 90, 100)
Do it again, starting with 90: (the smallest number in the unsorted part is 90)
You get 20, 40, 70, 90, 100 (the sorted part is 20, 40, 70, 90; the unsorted part is 100)
Since the unsorted part contains only one number, you are done.