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Yey! Thank you so much!
I will try to explain it as best as I can.
I would advice to use cotton as a surface, is the easiest one to paint on. Then you put it on a flat surface, like a flat piece of wood, and stick the t-shirt on it.

For the drawing, I advice you to draw the motive on a paper or to print the image you want to transfer and then use a "transfer paper" to transfer it to the surface.

There's also a way without the transfer paper. You take a copy of the drawing or a print and on the other side of the paper you paint all the surface with a soft pencil (3B is enough but the softer the better) then, you mark the silhouette of the figure on top of the surface you wanna paint on. Be aware to place it straight were you want it.
For the colour, we used low quality acrylic paints to paint over the transferred lines.
I think this post explains quite nice, you can see what I mean on his 4th photo.

In the end you should iron it upside down.

Please let me know if there's something that I didn't explain good enough or if you have any questions.
If you create something, let me know! I would love to see the result.
I tag @thisismylife because maybe she also wants to do it with her daughter :)