Photoshop Tools Used
Step 1: Sketching
First of all, you need to practice about anatomy so that there will be good proportions on your drawing. Use lines, circles, squares and other guidelines to make the sketch easier to draw before lineart. Start with this scaly arm. You may use an arm anatomy reference from anatomy books or other sites.
Burn Tool (Highlights)
Hard Round 1 Pixel

Flow (40% - 75% adjust)

Brushes>Other Dynamics (check)>Exposure Jitter (adjust)
Step 2: Inking
In next step, do the lineart. Use brush tool to darken appropriate sketch lines on different layer. But, before finishing the lineart, you must use Liquify tool to adjust muscles and outlines, or redraw using brush tool and eraser with keyboard shortcuts. This lineart can be made for anyone to color by themselves. Be careful on inking!
Brush Tool
Hard Round 3 Pixel
Hard Round 4 Pixel

Flow (100% adjust)

Filter>Liquify (for line repairs)
Step 3: Separating and Fading
Furthermore, merge down the lineart into  white space on the background layer. Use magic wand and separate the design into another layer, and then delete the background layer. This whole step vanishes hard, strong lines on the arm whenever the texturing is ready. (If the selection "invades" through the design, deselect and connect the unclosed lines together)
Magic Wand Tool

Image>Adjustments>Levels... (Increase the black arrow in Output Levels to fade the design into white)
Step 4: Texturing
Now it's time for putting the scales! Use the brushes as shown from the right. Put circles, ovals and ellipses around the surfaces. You can make any looping lines and curls. Draw smaller scales on where the muscle level is far from our view, and draw larger scales at the nearest view. You can use the same brush to darken between muscles.
Brush Tool
Splatter 14 pixels (adjust into 3-4 pixels)

Flow (40% - 60%)
Step 5: Shading
Erase the excess outlines. Darken the entire arm until it is greyish-black. This is a good direction to make scales more realistic before highlighting. Shade the muscle levels.
Burn Tool (Highlights)
(Any brushes and any size)

Exposure: 15% - 70%

(Decrease the white arrow in Output Levels to darken the design into grey)
Step 6: Highlighting and Final Darkening
After all the parts are darkened with correct amounts, lighten up the scales where the light comes from (top, bottom, side, front, back, or any direction) so that it casts on scales correctly. Make larger light casts on muscles that is exposed to the light, and make smaller light casts between shadowy muscles. Finalize darken the shadows.
Dodge Tool (Midtones)
Hard Round 2 Pixels (Use a larger brush size to lighten dark scales.)

Exposure: 100%

Burn Tool (Midtones and Highlights)
(Any brushes and any size)
Step 7: Adding Color
Use Photo Filter to color the whole arm into any color if you want. Do not use Color Replacement Tool for entire arm because the color you choose will slightly ruin the shadings. 
Filter>Adjustments>Photo Filter
Check "Preserve Luminosity", and select "Color". Use tint between green and yellow and adjust into greyish-green.
Step 8: Finishing
Put strong highlights and shines on the scales in where the strong light comes from. In distant scales at the edges, use Dodge Tool with a small brush and make very small looping lines as shown on the left. If you want to put more colors on green scales, (tatoos, reptilian colors, etc.) you may use Color Replacement Tool or Brush Tool with any modes.   
Dodge Tool (Highlights)
Hard Round 2 Pixels (Use a larger brush size to make bigger highlights.)
Brush Tool (Any mode for colors)
Color Replacement Tool (Use "Hue" mode to protect your shadings. In "Color" mode, use grey-shadowed color from Color Picker)