I made these onesies years ago (way before I started blogging) for Avery.   It was a great way to reuse my husband's "dam" shirt, that he could no longer fit (see image below to get the joke).  It took me a while to find the original tutorial that I used but I this is it: 
http://thismamamakesstuff.com/tutorial-rockin-baby-gown-t-shirt-refashion/.  While I was searching for the instructions I used I found this one: 
http://knitnatak.blogspot.com/2012/01/t-shirt-onesie-tutorial.html I like this one because it uses the original tshirt collar.  The first tutorial has you sew cut out the original collar and hem it with a zig zag stitch, which I found to be rather challenging.  If you use the first tutorial again I would put bias tape on the colar to make it look more finished. 
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| Alexis's dam shirt now. | 
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| This is another one of my husbands shirts that he wore back in his youth. | 
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| The gown is getting on the short side for Alexis,  maybe I'll cut it shorter and make it into a baby t. |