I think either way is fine, it's really your preference. Measuring however is critical. I do the sides first mostly because I want those long pieces to be long enough. There's nothing more frustrating than getting the top and bottom on only to find out that your yardage is short and you'll have to add to the length to make the sides long enough. lay out your quilt as flat as you can get it, smooth it out from the center in all directions without pulling it out of square. Then measure like I've noted with the red lines. If all measurements are the same and your cross corner measurements match, then your quilt is perfectly square and its safe to cut your borders, I never cut anything until I've thoroughly measured everything. Re-measure with every step. I usually add 1/2 inch to whatever I need to cut, especially if I'm laying the quilt out on carpeting where it's a little tough to really see if it's laid out completely flat. I use a carpenters measuring tape from the toolbox. have someone help you so you make sure the tape is right on the edge across from you, it can be hard to see.