Simplicity 3515 is an easy dress to put together. Maybe a little tougher to get a good fit.
I made a straight size 8, but lengthen the dress to a size 12. The size 8 is about one size too large for my daughter. I saw complaints of the gapping armhole on patternreveiew.com, but it may be all due to the top section just being too large. From just glancing at the pattern, you don't realize the belt is about a 6" section sewn to the front with the ties sewn under it. Well, I am sure that my daughter is not the only girl who doesn't stand still and the belt sags in the back. This could quickly be fixed by making the front belt section longer to about the side seams.
I did change a few of the construction steps. I wish I had taken a picture to show because it worked well. Maybe I can explain:
1. When sewing on the front waist band piece. I pinned the sash ties down in place and then pinned the center piece. To make this a pretty top-stitch seam, I tucked under the ends first (these are the ones on top of the ties.) Then I folded the top and bottom of the waist piece under. When I got the the corners, I tucked it completely under the sash ties. This left a smooth top stitch area.
2. I sewed the yoke lining on first to the dress. Then I stitched the neckline seam and finally topstitched the front yoke to the dress. I top stitch much better what I can see.
The main fabric of this dress is from Fabric Finders. It is a fun summer print that has been out for a few years.