I'm on photoshop, and that's all I've ever know, so sorry if you're on a different software. Hopefully it translates somewhat. (and if you don't have any software, there are free options out there, such as photopea, a free browser program)
The basics
Getting the scans you want is usually easier if you have access to the digital comic, through whatever means works for you. (mine is Comixology) But if you can't get your hands on entire comics, googling covers and preview pages are your best friends. Some of the early preview pages sometimes come without the dialogue added in as well, super handy!
You then drop the image into your image software, copy it, and paste it onto a new, square canvas. (I usually work at 200x200, unless I'm doing a lot of complicated blending or removal of backgrounds) Doing it this way allows you do drag around/resize/rotate the image until you find a crop that works for you.

Once you have your crop in sight, it's time to remove unnecessary clutter, be it speech bubbles, other character, distracting background illustrations, or borders. In this particular example all I needed was negative space, nice and easy. You can use your dropper tool and then paint over the remaining background with your color of choice, or use either smooth or clone stamp(useful if there's nuances/texture to the background).

Now it's time to color that sucker! First I put down two layers of Vibrancy, set to 100%. Vibrancy is my BFF when it comes to most comic icons.

Following that, two separate layers I've painted some colors on, set to soft light 100%. (the white bg is only so you'll see the colors I painted)



I then copy+pasted all the layers twice, and set one to invisible for later. Gaussian blurred the visible one, and set it to soft light 34%.

Added two bw textures set to screen, masking away/erasing a few bits that covered the subject a bit too much.

To finish it off, I dragged the copy layer we made earlier to the top, sharpened it slightly, and set it to soft light 59%.

In the end, my layers looked like this:

I'll have some tips & tricks up later =) Would obviously love it if anybody else wanted to post about their iconning process as well, and if you're stuck on something, ask away!