@Tamlin,
I think that what he means is that he is making his/ her own brushes and that they have no transparency.
What the problem is is quite simple. If you scan in an image, it automaticly gets an white background. If you make a brush of something with a white background, than that whit becomes part of the brush. And therefor you have a sollid square brush with no transperancy.
To solve this problem, you have to select the doodle, cut it and than paste in onto a new layer. I think the best way to do so is using the "magic wand" tool. Its in your toolbar. With it you can select the doodle by clicking on it. If it doesn't select the whole doodle, use "shift + click" to add things to the selection.
After you selected it,
"Edit > cut" than
"Layer > New Layer", and finally
"Edit > paste", dont forget to select the new layer before pasting
.
After you have pasted the doodle onto the new layer, simply delete the background layer( if it is locked double click it first). Than if you use that to make a brush, it will have transparancy, and will therefore blend into your images.
Hope this helps! And I know its a lot of text, but read it all....then you will understand the problem better!