If you only want to do minifigs, you're lucky, those are relatively painless.

You also don't need a template, you can just use an image of a regular torso (for example) as an outline, I think.

Step 1: Create the image. You don't need gimp, any image editing software will do. I'd personally really recommend a vector based software (instead of a pixel based one like gimp) so maybe something like inkscape or canva would be better. Both are free as well. Make sure to export with transparent background and in a reasonably high resolution- you don't want to have your figures all pixelated.

Step 2: Import the image. When you open part designer, there should already be a preset for minifigs. You don't have to use it, but I'd recommend it. Here, you can click on the part you want to decorate and add the image you've exported. I think you can also scale it, which would make the template obsolete. If you can't, it's mostly a matter of trial and error to get the size just right.

Step 3: export the part to studio. There, you can find it in the "custom" palette.

Done. :)

Part Designer comes with a minifig template:

https://studiohelp.bricklink.com/hc/en-us/articles/5877834417303-Creating-a-decorated-minifig

What exactly are you looking for? Custom as in you get to create your own? Or custom as in they make their own designs and you can buy them?

In the US I really like Citizen Brick and The Minifig Co. I also like Bricksanity over in the UK. There are a bunch out there, though, those are just the three I have bought from and am big fans of. They use authentic Lego products and print their own designs. They will also do custom orders of your own design, but I have never done that, but I know those aren't one-offs, they are large orders.

I get good results using Google.

Minifig decals are drawn by artists on programs like Photoshop. No website can replace an image drawn by a proffesional.

To be honest I am not entirely sure what are your expectations. What are you specifically looking for?

I used inkscape (it's similar to adobe illustrator, but free) to design the torsoes (complete beginner) and then minifigs.me to have them printed (5£ each). The results are here. If you have more questions or want more specific info you can dm me or ask here

You could use Illustrator or another similar program. Then get them printed on a sticker. It might be pretty expensive though.

ebay i assume. that’s where i got mine.

edit,, i think you’re probably talking about create a minifig aren’t you? or are you talking about custom tony stark etc

Flagship lego stores have (or will come out with) machines that you can print your own mini fig that’s customizable. I remember seeing it on the what’s coming up lego this year live stream they did. I don’t know what stores have it though. Do a search on you tube for

#LEGOMinifigureFactory #LEGO

I made my own LEGO Minifigure at the LEGO Store!

And a video comes up with some guy doing their own video on the process.

Reminds me of Big Bang Theory episode where they bought a 3D printer to make action figures of themselves.

Lol Bricklink has every part. Go there it is amazing.

Figure out which minifig parts best represent your friends and use bricklink.com to order parts.

You can order parts off pick a brick on lego's website or you can check the pab wall at lego stores.

Here's what I do. I have wishlists of very specific parts I really want to make specific characters with. I have other wishlists of more generic/general parts that I didn't have a strong idea exactly what character I want make with them. If one of the specific parts is rare or pricey I'll check shops selling that part for the best prices, and I'll usually order from the shop that has the most parts available from the other list.

The build a Minifigure towers at LEGO stores often have good parts. Used LEGO stores like Bricks and Minifigs. Maybe yard sales or eBay.

are you a Lego purist cause alt bricks/knock offs are the most affordable.

quality varies but if you just need 10 hench.msn with swords, might be an option.

Bricklink and pick a brick is a great place to get pieces individual pieces that you'll need especially if you already know what you want. If you're looking to get stuff in mass, which is what I prefer as well, you should look for places that sell bulk Lego parts. Stores like bricks and minifigs sometimes have a really good selection of parts in their bulk area. If you don't wanna spend the time looking then I would just purchase some bulk lots on Facebook marketplace. You'd be surprised what you could find in there and if you're lucky you can hit the jackpot and find a collection that's full of minifigs and accessories. I source most of my used Lego this way.

This subreddit, and the many regular posters on here (including myself) make a lot of different types of DND related minfiigs or builds, so it's a great place to get some inspiration for your minifigs. Good luck finding what you need!

Inkcups makes very good machines

Bricklink.

You’ll end up paying a lot for shipping and to meet minimums, but that’s where to go.

If you figure out how to use the wishlist feature you can find shops that have more than one piece you’re looking for.

Good old standby:
http://baseplate.com/toys/minifig/