Steps

Part1: making the skin

1-If you are retexturing a skin try to improve details very much and make it clear.

2-If you were working on a retexture and the details were broken [But they exsist] Try to high to contrast a very small degree [from 3-7 degrees ONLY] so it wont look buggy and would look detailed.

Part 2 Making screenshots it i-game:

1-First of All TURN OFF YOUR HD for [Options/videa]
2-Use your highest reslution you can squeez from your pc.
3-If you want a good and simple photo capture or video capture program you can ask for fraps its free and no trail dated mode.

4-Take the screenshots of the gun like ak47[An example]
On diffrent animations.

Reload animations: gives a better look on the weapon textures[Reload is the an important screenshot between magout and mag-in steps]
Idle displays the normal weapon.

Last step: take screenshot in brighty places without change game brightness.

Epic part3 making render

There are 10000000000 way to do it you can look also for tuts This is my simple way
1-Download Import and export smd for you 3dsmax
2-Import the weapon refrence Smd to 3dsmax
3-Choose it's postion [W_style or p_Style]
4-CLick at render in the menu bar at the top of the 3dsmax
5-and then click render
6-THen you will see your model ,use the model textures
7-click save icon.
8-Save as a .png format[Nothing else please]
9-Close 3dsmax and open photoshop
10-before moving to photoshop get any render background you want for anywhere.
11-go to photoshop and open the .Png photo from 3dsmax
12-the photo is transparent but the weapon is textured as you did.
13-Select All the photo by clicking CTRL+A and copy it
14-and paste it at your background using photoshop.

Uploading
1-Dont write any unuseful word in the read me or anywhere Like [VEry bad,Xd,LOl,...ect]
2-Publish your skin and accept any help b ppl crit for your skin.

Hope i helped alot.