Note: Click on the images to view a larger version
I'm just going to take you step-by-step through my personal process for drawing pictures with multiple characters interacting. Before you try this, of course, you should be reasonably comfortable drawing single character portraits. I'm not going to teach you how to draw characters, so much as how I put them together in one scene.
I don't have scans of the very first stage of this drawing, so two of the characters are already pretty much sketched out, but I will take you through the basic procedure:
1. Sketch in a basic "skeleton" for all the characters.
I do this because it lets you get some very good guide lines in, with minimum eraser time if (when) you have to make changes. My skeleton is VERY abbreviated, just a few quick lines. These help you work out where the limbs of characters are, their relative heights and positions, etc., with minimum fuss. Make sure the skeletons are as close as you can get them before you proceed to the next step. Relative size and touching/interacting characters can be very difficult, so get this step right! Don't forget to take a deep breath, close your eyes,and back up from the picture occasionally before looking at it. I have lost sight of the forest for the trees when I'm caught up in the throes of creative ecstasy, so a look at the whole picture is good every once in a while to make sure the entire composition is still good.
2. Sketch the characters' bodies
I've already sketched in Jaken and Rin, but Sesshoumaru was the skeleton in the last image. In this image I have fleshed out his body. I actually ran into a problem during this stage! Sesshoumaru's skeleton's torso was too short, and his hips too wide! I gave him a woman's torso and hip bones! This is why I draw the body like I do, especially for men, I always have the most trouble getting men's proportions correct. I always draw the characters "naked" first, a bare body outline, to help me render clothes, wrinkles, and folds realistically. If you generally go straight to details, you can probably skip this step.
3. Sketch the details
After the body is outlined to my satisfaction, I move to the clothing, hair, and other details. This is usually very time consuming, but not too difficult if I did steps 1 and 2 correctly. This is where a source is most important for fanart. Other than generic basics such as whether the character is male or female, and general body type and height, the previous steps don't require much checking to make sure your picture will jive well with the original character design. I will sometimes get ideas for emotions, or general feeling for my pictures from the original art, but I never actually consult character designs or original images until this stage. Sometimes my fanart is close to some picture from the original anime, but this prevents me from every actually copying someone else's art.
4. Ink over your sketch, erase the sketch lines
I think this is rather self-explanitory. I outlined Sesshoumaru's kimono details in red ball point because they're really too small to do in black, and then color red. Color pens are very nice for something like this!