Understand that the two digits of a two-digit number represent amounts of tens and ones.
Understand place value with units and tens. Represent numbers 0-99 with place value blocks. For example, given the number 72, the student should choose 7 ten blocks and 2 unit blocks. Working the other way, given 3 ten blocks and 1 unit block, the student should deduce 31. Conclude by giving your students the challenges found here and here, in whatever order you like.