Hello
I am in no way a expert at this stuff. Been mudding throught it myself.
If you want to use your sister's drawings you'll need to scan them into the computer. Then using a program (corel draw. illustrator or a freebie like gimp or inkscape) clean it up and add color etc.)
Then you print it out on to your transfer paper. And depending on the shirt color is what will decide what transfer paper to use. For darks you need a opaque paper.
Then you cut it out and then using your heat press transfer it to the shirt.
You can use a inkjet printer using pigment inks. THe epsons are the most recommneded ones for this , with the c88+ being the less expensive one.
You can use a laser printer and use laser transfer paper. The Okidatas are the most popular for these. Laser does not need to be cut before pressing on to the shirt.
Another method is called plastorial. You send your design to a company that makes these and they screen print them onto heat transfer paper and you then just press them on to your shirts. I haven't done this yet. Suppose to be good looking and not fade or crack. More per shirt but suppose to be better quality than transfers. Also need to normally purchase a minium number of transfers (at least at the companies I have looked at.)
Another way is
DTG (direct to garment) printer. You'll need about 15-20K to buy one. Or you can have a
DTG company print them out. Suppose to be a great looking way to go. However they have their issues too. I think to do darks the
DTG has to be able to print white ink which I don't think all
DTG's do. (I haven't done this either so may have some of my research mixed up in my head.)
There is also vinly. Think you'll be limited to maybe up to 4 colors at the most as you can't really lay the vinly on top of each other too much.
Last but not least there's the screen printing method. Probably what your most familiar with.
Good luck and hope I helped somewhat. Sure if I am wrong on anything someone more experienced will straighten me out.
Good Luck
Mark