HTV has its qualities and limits.
My favourite is that it is the same, may you print it once or 20 times: there is no minimum quantity because there is no mould / frame to print from.
However, in cuttable HTV you are limited by the complexity , the fineness of the design and the number of colours (I set the limit to 3). All colours are flat tints (so no gradiant effect).
How to design for HTV
The easiest way is to take a Sharpie and trace your drawing.
Less than 1 mm is risky to cut - and frigging annoying to weed (that means tweezing out the blank parts from the full sheet of colour).
If you draw with colours, use only flat tints, no gradations.
Maximum width is 30 cm for the cutter I use (Silhouette Cameo) - I can cut up to several meters long. To get an idea of the dimensions for T-shirt printing, look up the Dedicated Info Sheet.
Send me a high-resolution picture of your drawing with good light (white background no shadows) and taken from above.
Once turned into a vector-image, I can easily change the size of it.
How to prepare a file for HTV
My software can open .SVG files, but the easiest is a high-reolution .JPG or .PNG in black and white (or max 3 colours + background).
I will vectorize it myself.
Designs found on the Internet
Copyright infringement is a big risk, so I won't take designs that are licensed.