Hmm... okay, here's how I did it...
1. Bought some labels (25x33 mm). Got them from Pearl in Germany (VM-5205)
2. Measured the stickers and the SMCs, so I know where I should place what.
3. Created a template in Corel Photo Paint (you can use whatever graphic tablet you like), with marker lines for the different parts of the label (back, front text, picture).
4. Googled for the needed graphics and designed the labels.
5. Printed the finished stuff onto the labels and sticked them onto the SMCs.
You can download my labels if you want.
(Note: There is 1mm oversize to each edge, this means: My label templates are 27x35mm big).
All my labels are using 300dpi.
Atari, C64
fGB32
fGen32
fSMS32
Games
GPEngine
LittleJohn
OSNes9x
Note: Some SMC slots of GPs are pretty tight. The cards with the stickers don't fit in pretty well in one of my two GP's (well, they go in, but I have to be careful as it will damage the sticker). I don't care as I just open it and cut a bit out to make the slot wider.