Men's dress slacks are quite easy to alter. However, enough material must be present and visible to let out the waist on men's slacks properly. The center back seam facilitates this process. Practicing the steps used in letting out the waist in men's dress pants can help you do the alterations with ease.
Remove part of the belt loop of the pants so the waistband's inside center back parts are visible.
Remove the belt loop's stitches from the bottom, as well as those that attach the loop to the pants. Cut off all broken threads.
Open the waistband fully. Observe how much seam allowance you have on the side of the waistband.
Mark your stitchline. Use a ruler, a marker and a group of aligned pins.
Observe the horizontal seam that connects the waistband to the pants. Ensure that all seams are even.
Stitch along the markings made in Step 4. Use a heavy-duty needle. Taper into the existing seam line and move slowly.
Let out the seam according to your markings in Step 4. Taper it according to the length to which you let out the seam. Remove the old stitch line and pick out the old threads.
Review your work on the right side of the pants to see that the seams match. If they don't, rip and redo the portion that's out of line. If the discrepancy is minor, leave it alone; the belt loop will hide it when a belt is worn.
Take your finger and press the inside seam open. Fold it downward, ensuring the evenness of the top edge. Make sure your seam allowance is flat.
Iron down the waistband. Press the center rear seam open. Turn the pants onto the right side and iron the new seam.
View the seam line. Test it with the iron. If it presses out with no visible trace of the original line, you've done it correctly. If the old line is still visible, take it to the dry cleaners for steaming.
Stitch in the depression or "ditch" of the seam where the waistband meets the seat. Don't allow the sewing machine to catch the fabric on the waistband. Inspect your ditch stitchings. Make sure the starting and ending points from the center back seam are the same.
Return the belt loop to the pants. Start with the bottom edge. Sew it with the right sides together on the bottom edge of the waistband.
Attach the top edge of the belt loop. Fold it back the way it was before you took it apart. Using a straight stitch, go back and forth multiple times. Make sure you have enough slack in the loop for a man's belt.