I'll get you through this. I just went out in my garage and did one with pics for you. Can you tell it's raining here?
First off, I'm assuming you have the inner part screwed out of the outer part. If not, that's the problem. Anyway, here's how it's done.
I like to sharpen the end of the hose a little by trimming a little of the outer cover off. This makes it much easier to get it started on.
Clamp the hose snugly in a vice, but don't crush it. Leave just enough exposed to screw the fitting all the way on.
Use a ratchet and socket to screw it on, pushing while turning (left hand thread). Stop when it bottoms out.
Now clamp the fitting in the vice and screw in the inner part. You need to push in fairly hard to get it started. I like to put a little oil on the hose and fitting to make it go in easier. Screw it in until it bottoms out and you're done.
I have had them go on nearly all the way and get so tight that they feel like they are going to break or strip out. I have used them within about 1/16" of bottoming out and never had one come off.
This can be done with wrenches and without the aid of a vice, but it's like nailing jello to a tree.