We've all had to do it at some stage, and it is a right pain to do. However, sketchup has an inbuilt feature that makes it really quite easy. This tutorial will show you how to paint irregular objects easily, and make something that looks like this
Irregular surfaces tend to curve in several directions, such as the example surface below which will be painted. If you just click on the face of the surface, you'll end up with your texture not pasting properly and the surface being covered with fragments of your texture.
So the first step is to draw a rectangle in front of the surface you wish to paint. It should be the size of the largest edges of your irregular surface.
Now, paint your texture onto the rectangle
And use the position texture tool (Right click - Texture - Position) to make sure it fills up the entire rectangle.
Now, once you've done that, right click on the rectangle again, and go to texture - projected.
Now, select the paint bucket tool once again and hold down ALT while clicking on the rectangle. Then, click on the irregular surface. You should see something like this
Erase the rectangle and any leftover lines, and the job is done.