I always have a bunch of layers to keep it all neat. Otherwise the lines look messy :S

Tools used:

pen
curve
blur
airbrush
select
deselect
magic wand
bucket-fill
tablet
(you should also have a basic understanding of layers, layer clips/masks, and opacity)

Basic Steps:

Sketch
Lineart
Colors
Shading
Lighting
Background

1) First I draw a sketch on a couple of layers. After I finish sketching, I combine all the "sketch" layers together to avoid confusion. Sometimes, I instead do a sketch on a sheet of paper and scan it , but when I want it done faster I sketch digitally using the pen tool.

**NOTE: Don't be afraid to use a billion layers. Layers are REALLY helpful, as long as you name them so you don't get confused.**

2) Then I draw the lineart with different color contours. Sometimes I draw the hair or outfit lineart on separate layer so it's easier for me to distinguish.

3) I create a new layer and place it above all else, and I decrease the opacity of the layer containing the sketch and our new layer.

4) The vector tool is a VERY, very important tool to create neat, perfect lines. Instead of making another basic layer, click "New Linework Layer" and some different tools will pop up. Click on the "Curve" tool. It should look like this: ryuzaku.files.wordpress.com/2008/10/sai.jpg

5) If you go beyond the boundaries or your line is too long, you can hold Ctrl, Alt, or Shift and click to edit the lines. "Ctrl" lets you move any of the dots the line consists of ONE dot at a time only. "Alt" lets you delete any dots you click on while holding the key which is good if there end up being too many dots. "Shift" allows you to move the whole line between both end points or move it by either end point and the other one stays anchored. You can also use the color tool to assign a different color to your lines, but I usually don't use that.

6) I do the lineart in many layers, like the sketch, so it is easier to remove without having to carry the lines that are important or not do.  Hair, eyes, lips and eyebrows are left in separate layers, and sometimes the skin.

7) Lineart can take me a while since I use different colors for every item and try to create absolutely perfect lines

8) Color-layers should be placed between the sketch and lineart. Coloring is the best and easiest part for me. PLEASE PLEASE PLEASE MAKE SURE THE COLORING LAYER IS UNDERNEATH THE LINEART NOT ON TOP. Or else your art will look very messy.

9) For coloring I use the "Magic Wand" tool. Your lineart must NOT have gaps or holes or it will bleed out. Make sure your Target is set on "All image" and Detection Mode is best set on Color Difference and try to put the % at about as high as possible, but not too high where it will select everything. You can look up tutorial on "SAI Magic Wand tool" if what I said made no sense

10) You can use textures on clothing and accessories, which I love to do, by clipping layers. You can see a tutorial about using textures in SAI here: fav.me/d3xxyen

11) Lastly, I usually save the file as a .PNG without all the layers combined. (they don't really need to be, especially if you might go back to the drawing and revise it.)

So I usually take 2-5 hours per a full-body fully colored drawing, depending how detailed it must be and if it's a transformation or not.

Other Helpful Tutorials

fav.me/d33invl
fav.me/d4buvb1
rueme.deviantart.com/gallery/9…
fav.me/d2qv0kx

Hope this helps ! If some parts made no sense, don't be afraid to ask me :3