Ever since we first made a single player mod, people have written asking how to add a feature or make a change. It's not hard, but there is a steep learning curve.
I've always resisted explaining to people how to do these things as it would take ten times longer than just doing it for them and making a video would take even longer still (so don't be surprised that no one's going to make you a video). You wouldn't expect someone to just do it for you, so why on earth would you expect them to expend a heck of a lot more effort making a video for you?
After all, what do we get out of it? If someone was to help you in the way that you would like, that would have to take at least several hours out of their life to make and upload a video just so you can do one little thing and learn nothing in the process. And seeing as you wouldn't have learned anything, the next time you want to add something, you'll be right back here asking for help again. So I don't tell people how to do stuff and instead just point them at the many resources, like Armaholic, that provide ArmA editing tutorials and help.
99 times out of 100, I never hear back from them. On the rare occasions that someone does get back to me, they've learned enough to be able to do what they want and much more. That's where the mod authors on these forums come from. They've read through a pile of tutorials and figured out how to do stuff. They've often discovered that their initial request was actually more complex than they thought it would be. I haven't showed a single one of our authors how to do anything. And now they all know more than I do.
That's enough grumpy stuff. I know you didn't mean any harm by asking and I know you think your request is quite simple and reasonable. So let's see what we can do to help you out a bit.
Now, let's see. You'd like to add a base. That's not so hard. First of all, you'll have to decide whether you want an existing building (or set thereof) or something new. I'd recommend the former. You'll have to take apart the PBO files and figure out which ones contain the actual map files and buildings. Then you'll need to take the right file into the editor, place your buildings, save off the files, rebuild the PBO and away you go. That's about as easy as it gets. Once you've figured out how to do that, you'll be able to place any building or object on any map.
You may find that the objects you want to place aren't actually available in the editor (not all of them are). In which case, here's a little addon that will allow for all of them:
http://www.armaholic.com/page.php?id=6194If you have a go at this and get stuck, post on these forums. The chaps on here tend to be a lot more responsive to a specific question ("I get a 'mission.sqm, line 1162' error when adding a building") than to a more general one ("I want to add a building").
Seeing as you asked nicely though, here's a link to a video that shows you how to get most of the way through what you want to do.
https://www.youtube.com/watch?v=GHL78kbiwWM