For transfers, there are two types of paper available - one is designed for ironing onto white fabric and the other for dark fabric. The white version works by printing in mirror-mode. You have to prepare and print your image as a mirror version of your end result - then you simply place your printed image face down on the t-shirt and iron over it.
This worked pretty well, and I made three animal-themed shirts, all of which went down very well with the microbe...
The dark paper works differently. You print in regular (non-mirror) mode and then peel off an (almost impossible-to discern) backing sheet before ironing the image onto the shirt through a layer of silicone paper.
I'm afraid to say that this stuff was a total flop for me. It seemed like I had to hold the iron over it for a decade before the image would think about adhering to the t-shirt and, even then, it transferred so thinly that the colour of the t-shirt showed through. I don't know what I did wrong. I was sure I'd followed the instructions - but perhaps my iron is just not suitable.
After this failure, I had a much better idea - applique! I'd never done this before but decided to have a go using some of my Liberty scraps.
I used an approach that gives you nice neat shapes with no raw edges. As a first step, you draw your desired shape freehand onto a piece of interfacing. Then you sew it together with a fabric scrap, with the right side facing inwards, stitching all the way around your drawn outline.
As a last step, you cut a slit in the interfacing and turn the while thing inside-out and iron it flat, leaving a lovely applique shape with no raw edges, ready to sew onto the t-shirt. (I attached mine simply by zig-zagging all the way around the edges on my sewing machine)
I love the results of these applique experiments and I'd like to do more. Next time I might see whether it makes a difference to use iron-on interfacing, to get the applique fully adhered before sewing around the edges.