Open Illustrator and create a new A4 document and then select File > Place from the menu to browse to your scanned image. Place your sketch in the document; to resize it click on the it and select Object > Transform > Scale from the menu. Tick the preview box and change the scale until it’s correct, then lock the layer in the Layers Panel.
Create a new layer and name it ‘character 1’. Select the Pen tool (P) with no fill and a bright colour as the stroke (X). Start tracing your character, using a dot-to-dot method to draw the lines; moving the cursor will create curved lines. If lines cross, you can use the Scissor tool (C) to cut part of the line by clicking on it and deleting what you don’t want.
Repeat Step 5 to trace the outline for each character on a separate layer, adding a white fill to the characters once their outline is complete. To create rounded edges to lines, use the Brush option and change the Stroke as desired.
Once you’ve completed the outline of your drawing, spend a few minutes perfecting the lines. When that’s done, unlock one layer, click a part of the character with the Direct Selection tool (A) and choose a colour for the fill; you can change multiple colours by selecting shapes and using Edit > Edit Colors > Adjust Color Balance.
Begin to experiment with colour combinations; at this stage it’s not important to get the colours absolutely spot on as, you can easily amend these later. Using pastel colours is a good idea since vibrant colours can be distracting when you’re drawing lines.