You need proper coding skills first of all to script add-ons and you need modeling skills to make models for weapons/tools/vehicles/etc.
1. Open notepad
2. Code
3. Save as "(add-on type)_(add-on name).cs" without the quotes. For instance, the Biplane vehicle is "Vehicle_Biplane.cs"
4. Now you need a model for your add-on. You can make one yourself or have someone else do it. If you want to make it on your own, continue.
5. You'll need milkshape to model. I don't know how to model in milkshape so consult the "How to model" topic in Modification Help or whatever.
Also, we don't accept the excuses "its my first add-on" or "im new to coding/modeling". I'll warn you, the community is not lighthearted to unskillfully created add-ons.
