A while back I wanted to make a decal for my MacBook. The idea was to make an octopus that would sit on top of my laptop with some tentacles curving around the screen area.

I began with a pencil drawing. I roughly marked out the edge of the screen on paper as well as a void area where the corner was and figured out roughly how the octopus’ tentacles would work. Then I sketched out the octopus using my markers as a guide and scanned it.

My pen scrawls are not pretty, so the first thing to do was remove them. Opening the image in GIMP I used the Eraser Tool to remove the lines and the page creases to clean up the image so I could work on it more easily. I left some of the paper texture so I could give the octopus a distressed texture but you might want to get rid of all of it.
You can see there that the pen lines intersect some of the pencilled area so after erasing the pen some of the pencil lines will have to be put back in.

Here’s how the image looks after some minor cleaning.

I wanted to keep this image so I could go back and make changes if needed so I made a copy and placed it on a new layer above and called it “Outline”. It’s not a true outline yet because the white background of the paper is still present. To get rid of this, use Select by Colour to select the pencil colour and then Select>Invert to to invert the selection and press Delete to get rid of the white area. On this occasion doing it this way rather than just selecting the background colour worked best but depending on how your image scans in you may find you can just select the background colour.
Here’s how the “Outline” layer looks:

To start adding colour I made a new layer below “Outline” and began colouring using the paintbrush tool. You’ll find having the above Outline layer visible helps at this stage! To blur the lines between the two colours I used Smudge . I’ll show you how it looks with and without the outline below:

When you scan a pencil drawing it tends to look really washed out or else the line is dark but the paper is also dark. I find it easy to start with a light image and darken the lines later if needed. To darken the Outline layer go to Colour>Hue/Saturation… and reduce the lightness.
You’ll find that darkening the pencil lines really enhances it. Here you can see the difference. I’ve also added some texture to the colour with a speckled brush and a slightly darker shade. You could Bucket Fill with a texture or pattern if you had something less hand-drawn in mind.
Parts of the octopus, like around the curve of the head, are too sketchy for my liking so to add more definition I made a new layer above “Outline” and using the Eye Dropper Tool I selected the pencil colour and drew in additional lines and shading. It can be difficult to match the pencil size and strength so you may find you need to experiment a bit before it blends with your original pencil lines. A little bit more definition makes all the difference:

That looks almost ready to me. Now to finish cleaning up the image use a Mask Layer to white-out everything but the octopus.

Make a new layer at the top and colour the entire layer white (or the colour of your choice) - this will be your background colour. If you skip this step and have a transparent layer you won’t be able to see your mask. Choose Layer>Mask>Add Layer Mask and choose White (Full Opacity). The mask colours everything the colour of the layer, except the areas where you paint. Think of it like this; it’s like placing a piece of paper on top of your drawing. To see the drawing below you need to cut out an area to let it show through. That’s what we’ll be doing next.

For clean lines we’ll blank out everything but the shape of the octopus below.
First select your drawing. I found the Scissors Select Tool does a really nice job of this. Making sure you’re still on your new Layer Mask layer, choose a starting point on the outline of your image and then click along the outline to make the selection. The tool automatically fills in the edges between each node:

Once you’ve selected the image, press Enter to confirm the selection. Now use the Bucket Fill Tool to fill in the selection. Any colour is ok since this is just a mask and won’t show in the final image. You can be fancy and change the opacity or just go for full opacity if you simply want to cut it out. Now when you’re happy with the mask go to Layer>Mask>Apply Layer Mask to complete it.

You should now have your completed image!

Now for the decal. You’ll need to acquire some printable vinyl available from specialist photo paper suppliers. The vinyl will either have an adhesive or will cling statically - probably the better option for a MacBook decal. Check which side is printable and if needed flip your image so it prints out as intended. All that remains to be done is to print and cut out your image. Done!

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