Alright, I threw together a quick and dirty ragdoll pose to hopefully show you some of the basics. It's not perfect, lots of things could be done better, but i just threw it together to give you an idea.
First, spawn a ragdoll. Easy enough, just find the class you want in the menu and click.
It will appear flat on its back in front of you. This is the best place to start posing it.
Grab it by the center of the chest and rotate it towards yourself (I have this set to holding down mouse3 and moving the mouse).
Now upright, lower it to the ground until the feet naturally touch the ground flat. Freeze the ragdoll with right mouse.
Now just grab the head and move it around till it's where you want it. Best way is to hold down use and move the mouse, this rotates more precisely. Might take a couple tries and angles to get it just right. Once you get it, freeze.
There. We have something to work with.
Lets spawn some weapons for him to hold.
There now, we can't put them up to him or...
So we need to no-collide the grenade launcher. You can find this option in the Construction area of your menu. The grid there is an addon for my game, it won't appear in yours.
Left click on the grenade launcher. There. Now the two objects will clip together seamlessly, for better or worse.
Hold out and freeze the ragdolls hand and freeze the noclipped weapon in it. Hold use while moving the mouse to get the weapon exactly how you want it.
Use the finger poser to close his hand around the grenade launcher. Posing fingers is tricky, so don't worry about making it perfect till you're more experienced. Repeat with the bottle. Once they're good, you can weld the weapon to the hand and move them around.
He looks more bored than his new stance would imply. Now lets adjust his face with the Face Poser.
Be careful with the options, TF2 ragdolls can be overposed with horrific results.
Best to stick with the individual sliders. They're fairly explanatory, so find or build the expression you want from them.
Add a reason for him to look so happy...
And that's that. It's not perfect, but it's a quick and dirty happy-looking Demoman. Hope this helps, send me a Steam message and I'll see what else I can do