Help: Tips for sewing edges of a box shape Sewing Machines Parts and Sales in Los Angeles

Hello everyone. I plan on attempting to make a bag similar to this Simplicity 2274 Pattern, though I will be simplifying it and feel pretty confident that I can complete it without the pattern. I will be using a pleather, and adding in a separate lining later. Without the pockets and lining, the pattern is essentially only 3 pieces.

My question is, does anyone have good tips for sewing the body part onto the box ends? With a thinner fabric, I don’t think it would be too difficult, but since I have a thicker fabric, and only one shot at getting it right I’d be happy to hear your suggestions and tips. Thanks!