I have a stack of old T-shirts in my fabric box waiting to be turned into baby shirts (ones that are no longer fit for adult use but have enough good fabric in them to be worth chopping up), and this week made my first attempt, with an old Belle and Sebastian shirt. Lots of pictures after the cut.
I got some ideas from this Makerland blog post (though in this case I didn't need to use her shoulder yoke trick), and used Burda Easy pattern 9614*. The smallest size was 3T, which as expected is way too big (better too big than too small, I thought, for a first effort). He will grow into it soon enough.
Dismantling the T-shirt properly (I took apart the seams rather than just hacking it, to maximise available fabric) and cutting the pieces took half an hour or so. I was able to use the existing hems for the back piece and the sleeves, but not for the front as I prioritised getting all of the T-shirt picture in. It is worth carefully unpicking the neck ribbing so you can reuse that, too.
The actual construction was pretty easy (took about 1h20 all in, and I think I'd do it quicker next time). As jersey doesn't fray, I didn't bother finishing any of my seams properly on the inside, especially as I don't have an overlocker anyway. I did press as I went, though, which really does help it stay neat.
The neckband is a little puckered; I tried a bit too hard to stretch it as I sewed.
And on my slightly grumpy model.
(Note for my own reference: total time taken, from getting pattern out of the packet to finished T-shirt was 2h20 (done in several stages).)
For my next one, this post covers making a neckline that crosses over at the shoulders (a lot of baby shirts have this; it makes it easier to get their head through the shirt). I'm intending as well to cut the pattern down to 2T size myself, and make the long-sleeve version to have ready for next winter when he's outgrown his existing shirts.
* A irritated note in passing: nearly all the patterns, even ones for unisex clothes like T-shirts, have little girls on the front. Very very few little boys. Apparently one doesn't make clothes for boys? [sigh]