Here is a step by step guide to how it happens (using a real page to demonstrate.)
Step 1- Crazed Imagination- Daniel thinks of what is going to happen next (making sure it fits into the b ig grand running plot).
Step 2- The Line Drawing- Next, Daniel draws it out in pencil on a piece of paper (simple enough so far).
Step 3- Ink- Once Daniel feels happy with the drawing, He goes over all the lines in black pen, which gives us something a bit like this-

Notice the lack of backgrounds (I hate drawing backgrounds!)
Step 4- Life in Colour- Next, Julie gets out the magic prismacolour pencils and gets to work colouring it and shading it.
This then scanned onto the computer giving us something like this-

Obviously now it is starting to look more like a proper comic! Julie loves doing backgrounds so this is where they get filled in.
However, at this point there are still a few mistakes (the priestess' arm in the bottom corner you can see a line where it was corrected for example). At this stage the colours are rather light and washed out, there is no proper guttering between panels, and there is obviously no text.
Step 5- The Miracle of Technology!- The final stage is Daniel at the computer, going over the image, touching up mistakes or faults, adding guttering, darkening the coluors to make them boulder and addings the text so we can tell what everyone is on about!

Notice that the one colour backgrounds are added in on computer, so they have a nice clean look (and it saves a HUGE amount of time!)
Also, you might note that the edges are trimmed off as the scanner always puts a white boarder around standard size paper.
Well there you go, now you know how it is done.
If learning of all this might ruin the magic for you, you should probably try to forget everything you just read.