Men's clothes are usually bigger than women's. So any reconstruction that involves converting a man's top to fit a woman would usually need to be made smaller.
Specifically, narrower in three areas:
trunk (the upper body)
The only exception is the bust area. Most men don't have breasts, so they're usually made with straight squares of materials which skim over their chest, without calling attention to anything in the bust area. Men's shirts are shapeless in the front. Women's shirts, on the other hand, tend to have a dart or pleat under the bust to create that curve. Anyway, we'll need darts here to accommodate for those curves if you are sewing for a female silhouette. It will be easier to put in darts before you narrow the trunk.
Here's how to reconstruct that man's shirt to fit a woman!
1) Narrow the shoulder-to-shoulder width
2) Put darts under the bust so that shirt will curve over boobs
3) Narrow the sleeves and trunk of the shirt
Please read all instructions first :)