1. Start off with a nice blank 400x400 (or whatever size) canvas. Get your shape tool, and make a rectangle with whatever colour you want; you can change this later though, so don't worry about it yet.
Be careful through this tutorial not to rasterize the layer just yet - keep it as a vector shape. If you don't know what this means, don't worry - just leave the merging layers until later.
2. Now we're going to add some gradients. Select your gradient tool (G) and use settings similar to these:
note the type of gradient, and that the opacity can be lowered. Create this linear gradient on a new clipping layer perpendicular to the edge of your rectangle, like so.
To create a clipping layer, create a new layer as normal, and then hold alt and move the mouse cursor over the line separating the two layers in the layer box, as shown here:
The second gradient is optional - use settings similar to these:
and make a gradient covering the top half of the polygon.
3. Add a single pixel stroke to the edges of the rectangle by going Layer || Layer Style || Stroke. Make it whatever colour you like, ideally one to compliment the colour of the main body of the polygon. Again, this can be changed later.
This is the basic template we are going to use for all of our polygons. Go File || Save As... and save the PSD file as it is.
4. Open up a copy of your polygon so far. The coloured box next to the layer containing your basic rectangle indicates the basic colour of your polygon - double click this box to alter it. If you alter the main colour, you may also need to alter the colour of your stroke by right clicking on the word 'stroke' below your main layer and clicking 'stroke' from the drop-down menu. You can also alter the colour of your gradients by either colorising the layers (ctrl/cmd + U) or altering the blending options. The background can be changed too. Once your happy with the polygon, proceed to step 5.
5: Finally, we're going to transform our polygon into a more interesting shape. Link all the layers other than the background and press ctrl/cmd + E to merge them. Press ctrl/cmd + T to begin free transforming, which is the best way to do it as long as you are experienced with it. Otherwise, go Edit || Transform and play around with the transformation features until you have a shape you like. Try 'perspective' and 'distort' in particular.
When you're happy with that, turn the opacity down to suit, and the polygon is complete.
