According to Wikipedia, kitbashing or model bashing is a practice whereby a new scale model is created by taking pieces out of commercial kits. These pieces may be added to a custom project or to another kit. For the hobbyist, kitbashing saves time that would be spent scratch building an entire model. Hobbyists may kitbash to create a model of a subject (real or imaginary) for which there is not a commercial kit.
I think for many of us 1:6 military figure collectors, we would have kitbashed at least one figure or more because we want to upgrade the figure we have or customize a figure we would like to see but is unavailable in the market. This happens especially when a new collector comes into the market and finds that military figures of a particular era (e.g. Vietnam War figures) have ceased production and the older ones are no longer available. Dragon had released quite a number of SDU figures in the past with Hot Toys following suit but since then, if you want an SDU figure, you might have to resort to kitbashing one yourself from parts available. It's most easy to kitbash PMCs (Private Military Contractors) because they don't have uniforms nor standard gear.
So what do you need for kitbashing?
For a start, you will need a 1:6 nude figure
You might also want to get different heads i.e. 1:6 head sculpts for your bash unless you are building a clone army then they can all look the same. Back in the day, G.I. JOE and Action Man all had the same head sculpt. Then they tried to differentiate them by changing the color of their hair and eyes but they all still had their trademark scar on their faces.
Next on the list, the particular type of uniform or outfit for the 12-inch figure
And when you have amassed as much stuff as I have over the years, one luggage (that's 1:1 scale) won't be enough for all your clothes LOL
If you want the figure to be a military type figure of a particular country or era, besides the outfit or Battle Dress Uniform (BDU), you'll need the right helmet and boots to go along with it
Most importantly, every soldier needs a weapon; the more the merrier although in the real world, each weapon should have its sufficient load of ammunition to go into the ammo pouches, thus adding weight to the already heavy laden soldier so go easy on the poor fellow. Unless it's a video game character then they seem to be able to carry almost every weapon in the arsenal available to them plus an unending supply of ammunition and explosives AND they never tire ;p
Of course one should not forget the gear / loadout every soldier needs. 1:6 props can make for an interesting composition when it fits