You will need:
Pre Steps: Import the VTF file of your weapon/whatever into Paint.NET... I won't get into this as their are many tutorials, but this may be helpful.
NOTES (The main instructions will be in the pictures!)
Every weapon is different. Don't have the, "I will make this slider ELEVEN every time!" type attitude, because then something may not look great along the road.
Experiment! Whether it be with different effects or sizes or adjustments, don't be afraid to try something new as there always will be a back button.
Got a plugin similar to Sharpen+ for Photoshop/GIMP? Please tell me and I will put it up in the tutorial for those who use other programs.
Paint.NET and its plugins are always free! It is often considered the "Poor man's Photoshop", but I find it quite useful for what I do.