If you wonder what tools I use and how I usually draw in Illustrator - here is a little tutorial.
Firstly I normally draw sketch on the paper, using pencil or ink pan. Then I scan it(or make a photo if you haven't scanner). And then put into Illustrator, to draw.
If I want to create seamless pattern, to be more convenient I draw on the paper square, so I can imagine how to place objects and what of them will be out the artboard.
Open Adobe illustrator and press Ctrl+N - to create new document. The size of artboard depend on how much thin line I need to draw my illustration. I prefer it to be 1000px x 1000px.
Compare this 2 artboard sizes, and lines:
as you can see there is a big difference.
So, choose the size what is perfect for your idea and set it in settings. Press OK.
Then, put your sketch into illustrator and make it the same size as your artboard.
Then, in the top menu change the opacity of your picture, so black lines will not interfere you to draw.
And then, in Layers box lock this layer.
Next - create the new layer
Take the Blob Brush Tool - I always use this tool to draw my illustrations.
Click on this tool icon twice, so Blob Brush dialog box will appear.
In the Blob Brush Tool Options dialog box, you can adjust the following options(try every of them using different settings to feel how it work):
Keep Selected – keeps the contours selected when you continue to draw.
Merge only with Selection – This option specifies that new strokes merge only with an existing selected contours.
I recommend leaving these two options unchecked, or check only the second.
Fidelity - how exactly your lines will be drawn.
Smoothness - how smooth will be your contour.
Try them, set 0 and draw, and then in compare another numeric value
Then, you can adjust the Size, Angle and Roundness of the Blob Brush’s effects through a variety of parameters in the drop-down menu. If you have graphic tablet choose Pressure option, so all your settings will depends on pressure. For example, set Size 1 and Variation 1pt too, and choose in the drop-down menu Pressure. Tnen draw firstly with light pressure and then - hard. That is how it will looks.
Try to change Roundness. Set for example 40%. Look how your line changed.
Change Angle, and try, look what changes will it give.
Ok, set all settings as you like and make the line perfect for you.
Let's start drawing!
Enlarge the view of your artboard using "Ctrl+"(to make smaller press Ctrl- )
To change the size of your bruch use [ and ] keys.
Draw over the sketch repeating lines.
When all details will be ready select all using Selection Tool
And press Ctrl+G to group your objects
If you draw bad line you can press Ctrl+Z to back your las action. or select it and press Delete.
If you need to erase some elements - use Eraser tool.
Select the line you need to erase and use this tool.
To select object which is grouped use Direct Selection Tool(A).
Draw all other objects.
Now you need objects to create seamless pattern.
Here is the tutotial: http://yuliiabahniuk.blogspot.com/2013/01/how-to-create-seamless-pattern-in.html#more
P.S. It is very comfortable and quicklier to work using hotkeys. I found for you this pictute
and, besides, you can change in Settings hot keys and set keys you prefer.