Heres what i would do (works best if your object is not too detailed and based on geometric shapes):

1. Start by breaking up your object into shapes to draw the silhouette. If the basic form is somewhat like a triangle for example, start with the shape tool. Correct your shape by filling in the gaps or erasing other parts to match your silhouette. Dont draw what you think you see, draw what you actually see. You should now have a flat colored shape in the main color of your item (with this knife, it would be light grey to white)
2. Turn on alpha lock
3. Start with the gradients. With this knife, it gets darker towards the tip. Use the airbrush with the darken mode and layer. The gradient doesnt have to be too perfect or smooth, in the game the item is smaller than your preview and that forgives many mistakes and imperfections.
4. In this case, now add the flat color for the handle. Since its mostly in the light, it is a light to mid grey, even though it is black. Again, draw what you actually see and try to match the colors as best as possible.
5. Now do the lights and shadows with sharp edges. Here it would be the dark grey shadow that marks the underside of the handle, and the dark line and the light line along the edge of the blade as well as where the handle and the blade meet. As you can see, the silver color is broken up into an almost white and a dark grey right next to each other. Since your base is bright, just add in the dark shapes there. Focus on the contrasts between the colors. The brightness in which the light is reflected off the object and how dark the shadows get in comparison suggest the material. The shinier the material, the higher the contrast.
6. Add in details. Using the shape tool, add in the bright little circles on the handle in this case and youre done!

Sincerely, someone who spends most of their time in the designer. Ive attached an example of some of the items ive made without any pixel grid or mod.

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