Distribute a set of Base 10 blocks to each student. Ask them to hold up a unit cube ("ones" block). Ask them to hold up a rod, and tell you how many unit cubes equal a rod (10). Ask them to hold up a flat, and tell you how many rods make a flat (10), and how many unit cubes make a flat (100).
Draw a place value chart with three columns, separated by vertical lines, on the blackboard. Label the left hand column "Hundreds," and write "100s" directly below it. Label the center column "Tens," and write "10s" directly below it. Label the right hand column "Ones," and write "1s" directly below it.
Tell each student to copy the chart onto a piece of paper, and keep it for later use when learning regrouping for subtraction. Place the appropriate Base 10 block above the column labels. Demonstrate this for them using the paper Base 10 blocks, taping them above the appropriate columns.
Demonstrate that the "Ones" column can't hold more than nine unit cubes. If they have 10 or more unit cubes, they need to trade (regroup) groups of ten cubes for a rod. The rod(s) should be placed in the "Tens" column. Ask the students to imitate what you did on the blackboard as you restate the rule. The "tens" column can't hold more than nine rods. If they have 10, or more, rods, they need to trade (regroup) groups of ten rods for a flat. The flat(s) should be placed in the "Hundreds" column. Ask the students to imitate what you did on the blackboard as you restate the rule.
Work problems with the students until they can solve the problems without using the Base 10 blocks.
Tell the students that when a subtraction problem requires them to subtract a larger number from a smaller number in one of the columns, they will need to regroup by borrowing from the column to left. Demonstrate a two-digit subtraction problem requiring regrouping in the "Ones" column, such as 25 -- 7, with the paper Base 10 blocks. Write the subtraction problem on the blackboard next to the place value chart. Ask the students to imitate your work on their place value charts.
Place five unit cubes in the "Ones" column and two rods in the "Tens" column. Ask the students how many are in the "Ones" place in the number 25 (five "Ones"). Tell the students they need to subtract seven "Ones" from five "Ones," and ask if they are able to do it. Tell them they can if they regroup by borrowing a rod from the "Tens" column and trading it for 10 unit cubes.
Take a rod out of the "Tens" column and add 10 unit cubes to the "Ones" column. Now the "Ones" column has 15 unit cubes, and they can easily subtract seven. Take seven unit cubes out of the "Ones" column, and set them aside. Ask the students what is left after the seven unit cubes were subtracted (one in the "Tens" column and eight in the "Ones" column, or a total of 18.)
Continue practicing subtraction problems requiring regrouping by borrowing until the students are able to solve them without the use of Base 10 blocks.