Modding: When and how?

[Asheshouse]

As DD_crash says, make a copy of your existing IL2 folder and rename it something like IL2 Mods. Keep the "clean" version for applying future TD patches.

Mods are generally made to work in a specific game version. They may or may not work in other game versions. You will see in SAS, for example, that each mod has a game version icon, or icons, to indicate which version it is proven to work in. Map Mods may need to have additional Mod Objects or Ground Textures added first otherwise the map may have missing objects which results in missions being unable to be saved.

Installing a mod which is made for a previous game version may, or may not, cause crashes. Stick with mods proven to work for your game version initially.

If you are using v4.12 then the obvious choices are HSFX7 http://www.sas1946.com/main/index.php?board=188.0, for a large mod pack, or SASModAct5.3 http://www.sas1946.com/main/index.php?topic=37662.0 for creating a custom selection of mods. You will find more info on both of those options at SAS.

There are alternative SASModAct's UP3 Pack and DBW for earlier game versions. SAS also has a special section for v4.09 installs which continues to have avid support.

Do all planes,ships and objects mods need other mods to work?

Depends on the mods. Some do, some don't. The mod installation notes will normally explain what is required.
Read the notes before installing.