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That's pretty much what I do, with one exception: I pin the fly before the acetone bath.

Keeping wings open is difficult especially with small, soft flies. I have found two ways to keep at least one wing from folding.

Method one is slower and labour-intensive, but gives the best results: Remove the flies one by one from acetone and immediately bring one wing (or both wings, depending on position) in contact with a flat piece of paper. The wet wing(s) will stick to the paper surface and will neatly unfold when the fly is flightly moved on the paper. Let the acetore evaporate until the wing separates from the paper. Do not use filter paper - it will drain the acedone before the wings have time to unfold. Normal office paper works, slightly thicker paper is even better especially for smaller flies, where the size of the acetone drop carried with the fly is small. With this style it is usually possible to 'rescue' both wings.

Method two is not as good, but it is faster and can be used for larger number of flies. I use it regularly for agromyzids and drosophilids: Micropin each fly asymmetrically. Push each pin with a fly into a flat piece of plastic foam until one wing rests again the foam surface. Let the foam piece float in acetone (flies downwards) for a few hours. Lift the foam out and dry. Remove the flies when the foam surface is dry but before the flies have dried through. Some foams will melt in actone - test your setup before trying it with flies!