Lay the short sleeve shirt on top of the long sleeve top. Measure about 1/2 inch further out from the edge of the short sleeve top and cut the sleeve. This is assuming you’re working with a knit top. If you are working with a non-stretchy fabric you might want to do 1 full inch so that you have room to finish the edge or roll it under twice so it doesn’t fray.
Fold your long sleeve shirt in have length wise so you can light up the cut sleeve with the remaining long sleeve.
Trim the second sleeve so it it the same length as the first.
Iron under the edge of each sleeve 1/2 inch.
Hem the sleeves. My favorite way to hem knits is by using a twin needle with wooly nylon thread in the bobbin and regular thread in the needles. If you’re not familiar with this technique check out my guide to hemming knits post.
Marvel at your now weather appropriate garment!