Mix in cold butter by gently pressing pastry blender into the butter and flour mixture. Butter is cut-in when the mixture is crumbly and looks like coarse meal. This may take a little bit of work. You may have to use a knife to clean off the pastry blender once in awhile.

Cutting the butter in coats the proteins in the flour and helps prevent the gluten-forming proteins from joining together with water and with each other.

If you don't have a pastry blender you can use two knives to cut-in the butter. With knife blades close together, move the knives back and forth in opposite directions as in a cutting action. This will take more time, but it does work just as well as a pastry blender.