While creating this tutorial I listened to BMA Sessions with Dave Scotland: http://www.mixcloud.com/bmasessions/
Like on Facebook: https://www.facebook.com/BMASessions?fref=ts
Step 1 - Collect initial reference
First thing first jump online and find some reference images of the asset you wish to create. For this step don't worry too much about the quality of the images, just try to get a handful of different designs that you find interesting. We will collect more specific reference in a later step. Google images is perfect for this initial search.
Step 2 - Use the reference to sketch out the object.
Spend a little time sketching from the photos to get a feel for the object, the more varied your reference images, the more feeling you'll get of it in 3D space, and the easier it will be to draw your concept of it, as you will get a feel for what design choices you can make. If you're confident or have time pressure, then you can skip this step, but I find it very valuable step in the process and have noticed that the final image is often better where I have done this.
If you have difficulty drawing from reference (or real life) I would recommend checking out the tutorials on CTRL Paint under the 2.) Traditional drawing section (it may also be beneficial to go through 1.) as well), I think that the Drawing Shape: Contour and the Drawing Shape: Linear Block in will be of specific use to you.
Here are some sketches I completed while concepting for my major assignment, they were done in an application called Mischief (later ones in Photoshop), but you can use any program you are comfortable with:
Step 3 Thumbnail Silhouettes
In this step start sketching out ideas in black, I like to block out the shapes first and then refine and add detail with the eraser and a smaller sized brush.
Further Refinement and detail pass:
This stage is more about getting your ideas down (quickly) so don't worry too much about getting the perspective right at this time, as long as it conveys the shape you're after. If you doing this for a client (and if you are I doubt you'd be looking at this tutorial ;)) this is probably what you would first send (they will usually specify numbers) and they would select the ones they like for further development.
Here are the silhouettes I completed for this as an example:
I'd recommend checking out the more indepth videos of Silhouette sketching from FZD design school on youtube for instance these two videos:
EPISODE 30 Character Silhouettes part1
EPISODE 30 Character Silhouettes part2
Step 4 Reference
Now that you've made some rough design choices in the silhouettes its now time to collect specific reference for the assets, ones that will allow you to check values against, and that give you a good idea of the surface detail. These may even be from objects not directly related to the object.
Step 5 Value
At this point you receive feedback or choose those designs you find interesting from the thumbnails that you'd like to develop further. Here I flatten the silhouette image, and select a number of squares equal to the number you're going to develop, which in this case is four (one of each) but normally you'd do a lot more.
With that highlighted I then CTRL+C to copy the selection, and then go CTRL + N to create a new document, it should size to what is in the clipboard. I then CTRL+V to paste in the selection to get the lines and then erase what is there, although you could just redraw the lines. I then select, and copy and paste the silhouettes I want to work on into each of the squares.
Once you have the silhouettes arranged its time to resize it, which you can do through the image, image size.. option. Make sure to chose image size rather than canvas size as that option will add empty space to the image rather than scaling the whole thing. I multiply the size by the number of columns and rows you have pasted across, so in this case was 200% each way, but if you have more then resize accordingly.
Once that is in place I like to start looking and correcting for perspective at this stage, a good tool for generating the grid for this is Carapace by Epic games: http://epicgames.com/community/2012/11/free-art-tool-released-thanks-to-epic-friday/
To use select the part of the image and CTRL+C then open carapace and CTRL+V. Now try to locate two lines along a plane and right click drag to draw a line on each line of that plane, then CTRL SHIFT 1 to add a vanishing point where they converge, do the same for the opposite angle and then CTRL SHIFT 2 to add the second vanishing point. You may notice that your horizon line is skewed in one direction, use the mouse wheel to zoom out and then correct one of the vanishing points (left click drag) so that it is straight. once this is pretty right ctrl+C to copy and then back in photoshop ctrl+V to paste the grid.
Now drag that layer under your silhouette and set the silhouette layer to multiply so the grid shows through.
Repeat for the other images.
Note that for organic forms such as the trees and mushroom in this example, this isn't all that important, but I do it for this anyway as part of the example.
Also note where you have put the top or bottom of surfaces in the silhouette (even if they're not actually visible) as this will help determine where the horizon line should be. No top sides should be visible above the horizon line and no bottoms below.
It should look something like this:
Then use a combination of the transform tool and the brush/eraser to correct for the perspective.
I like using the warp tool (within the free transform to correct for this)
Mines doesn't appear to be too bad, so I've just tweaked it here and there.
You could of course leave perspective until the colour phase, but I find it easier at this stage as you have less to correct, you don't have to repaint a lot after adjusting.
Now we can get to the actual painting :)
I tend to start by adding the swatches palette in to the colour bar, and resetting to default so that you have the greycolour swatches, and then I select about the 90% gray, and start painting in the dark areas on a new layer as I like to work from dark to light. I'll work across all the images in this first and then move down the grays until about the 55%. Work off reference and this will be a lot easier. Also with man made objects with straight lines remember the shift to draw a straight line (horizontal or vertical) and the click, shift click to draw a straight line between two points.
Another thing that will need to be decided on before we get too far is the light source and its direction. For these I tend to keep the light pretty simple and just have it from one direction, for complete scenes and characters however it will get a bit more complicated.
As you're working try not to alter the silhouette too much, and keep in mind the perspective lines.
At 90% gray I try and just get a feel for the form, for the trees I mainly try to get the trunks and branches in roughly even if its going to be obscured by leaves, for the branches I will probably leave them at this value, unless of course its going to be in direct light.
80% Gray:This is were I decide on light source direction, and mark this in on the perspective layer so that I remember.
You'll notice on the mushroom I've indicated bounce light.
20% and 10% Gray
As I get closer to white, I change the background colour to a dark gray so that I can see the marks a bit better.
Step 6 Clean up
Now I clean up any stray lines (like those around the mushroom and birch tree) and carefully erase any black I don't want (like around the highlight areas) and I add a bit of a background to help make them stand out a little more. Last thing I do is add my signature.
And there we have a value concept, In the next part we will pick one of these and take it from the value image through some colour comps, adding a background and through to the finished Colour concept.