[header] How to stretch the waistline of your pants [title] Put on the pants. [step] Pull your pants on as far as you can, ideally to the point where you can zip/button them up. [substeps] Laying down while buttoning/zipping your pants can also make them easier to get into. 
If you can't get the pants on far enough to zip them up, first do a little manual pulling. Use your arm strength to pull and stretch the waistband enough that you can pull the pants on.