Step 1: Click the obvious subscribe button.
Step 2: Go to your world in 'load world', then edit the settings, then click 'mods', then include the mod you just downloaded into the right column.
Step 3: Ok and ok. Then play.

You might have mentioned the need to manufacture the ammunition.
Thank You for your timely response.

When I press "Subscribe" , where does this file(mod) is downloaded? I cant find it -_-

Oh, did I mention I really like this mod.

Really now, B1G_DAWG, don't be like that. The mod description makes no secret of the fact that these weapons require ammunition - hell, I even laid out the material cost for you, both of the weapon and the shell. Outside of that, it's no different to the already in-game weapon systems (correct me if I'm wrong here, but I think that to be fairly obvious) and so the usage should be intuitive. I've said this before somewhere - if you need to ask how to operate this mod, then you need to go back and get a firm grip on the base-game's core mechanics first.

@m0riaL™ - You will find any mods you subscribe to when you launch your game and open the mod list upon either editing a savegame or starting a new world. You never need to interact with the file itself, so knowing where it is saved on your hard drive doesn't benefit you.

@NinkaPirate thanks for info. I watched a video with a guy saying that you gotta turn to .zip the downloaded file , and then unzip it on the "Mods" file of SpaceEngineers. But if there's no need for that , im ok with it