We make a lot of Pua's clothes by altering baby clothes. This works for dogs and cats too. Once you get the hang of this you can find lots of cute things at thrift stores and make them into pet shirts. I do keep an eye out for nice pet clothes too but most of them are just not as cute as baby clothes.
This will focus on turning a baby Tee into a pet Tee. If you want to alter something like a hoody those are more difficult. Sometimes if I like the look as is I can just ad a draw string or elastic to the waist of the hoody, jacket, what have you. Other times I take them in by cutting out long triangles on each side to take in and reshape the waist area but I think that would be harder to teach. with hoodies i always tack the hood back since she wont be wearing it.
Anyway for the demo I grabbed a shirt I sometimes use on Quasi it's a bit snug since he's a bit chunky. This is an issue you need to address by choosing the right size baby shirt. As will be explained you should put the baby shirt on backwards so the front logo is seen on the back of the dog. It should fit comfortable around the back and not pull under the arms. The front will be loose but that's what we will alter. Since even in thrift stores they will frown on you trying the clothes on your dog it may take you a few tries to get the right shirt size.
How to make a pet shirt from a baby shirt 1
How to make a pet shirt from a baby shirt 2
What I forgot here in the photo is to turn the shirt inside out first.
How to make a pet shirt from a baby shirt 3
How to make a pet shirt from a baby shirt 4
To be perfect and leave no spots use a washable marker. I just didn't worry about it since it's the underside anyway and Quasi won't be wearing this much since, well, he's to chubby.
Only mark on the one side. Keep the shirt folded and sew on the line and the sides should remain even and leave a center seam. You may need to pin the shirt but I don't bother with that myself and just watch to keep them lined up.
How to make a pet shirt from a baby shirt 5
How to make a pet shirt from a baby shirt 6
Finished shirt from side
Finishes shirt from chest
This is the ant shirt I altered for Pua. You can see I cut out more belly area so her feet don't get caught up in it.
finished shirt front
When I did Pua's ant shirt it was even but I left extra chest when it's not needed on her. www.flickr.com/photos/tamanduagirl/4277244363/
Might take some time to work out special alterations to the pattern your pet may need.