watch some demo reviews. pretty sure the ones pyyyour did with slin talking in the background were really good.
https://www.youtube.com/watch?v=bUhj_Tk0stE

imo playing scrims with your team is the best way to practice 6s demo. Since you say you've only played pugs and lobbies, you should think about looking to join a team.

Training for 6s demo as far as gamesense goes should come down to playing it a lot in pugs with decent players, or getting a team to practice with. Over time, you'll start getting into the flow of what to look for at middles, what enemy players you can catch out of position by cutting off their exit route while your scout challenges him, things like that.

For mids, it's about getting damage on certain things to remove their positioning. If you hit a nice meaty sticky on a scout on a high-ground part of mid, he'll likely back off if your team is there to pressure him, which makes winning the mid a lot easier if your team has the right positioning to push into the remaining players on mid.

I've found that the downtime parts of a round (stalemates, pushing last, holding second) can be an opportunity for you to help the flank with a cheeky sticky trap or just general support. The map depends on what you should be doing, but generally you can focus on helping your flank in some way without leaving your pocket out to dry at the chokepoints.

Watch Marxist's videos. They go in depth about the general ideas of what you should do as every class, but focus on small details like where you should put your first two/three stickies on certain maps. If I were to spend more time on them I might not be shit at demo #200dpm. here's a playlist that will help you (or anyone who wants to get better at literally any class

Learn rollouts and practice them on a create-a-server with the stickyjumper/script for hours on end

Watch invite players review their own demos/review someone elses.

Don't play like Kaidus, he makes retarded decisions and gets away with them because he's got godlike DM. exhibit A

Play tons of pugs, scrims, lobbies, and whatnot. In terms of helpfulness scrims>pugs>lobbies but anything is useful

The demo on my last team was fucking insane because he spent tons of time on soapdm servers and got really good at hitting the pipe projectiles, he's the one who (somewhat) carried our team

If you want to practice your pipes, dm servers can be a viable option. However, if gamesense and positioning are your main problem, reviewing your own demos to indentify mistakes you are making. Pay attention to when you get caught out of position, what you die to etc. Getting someone else to review your demos with you can help to see mistakes you aren't aware of. Apart from that, keep playing pugs and scrims, you improve at the game by playing the game

experience and time is the best thing you can do, just always keep playing and over time you will improve, also the previously mentioned posts have good information aswell

also, i found this video to be extremely helpful: https://www.youtube.com/watch?v=kbD8yse2m0M

focus on hitting every. single. sticky. ever.

^ Pipes may be flashy but good stickies are what make the difference, don't get lured into practicing just pipes because people tell you that stickies are gay or what not.

oh and according to gasgara, you need to aim your rollers so they hit too

I would look into getting a mentor too once you find a team. They can really help you iron out specific aspects of your game imo.

Sometimes it's better to aim stickies by literally shooting areas and not people.
Watching demo reviews is great, looking over your own demos is really good.
Practicing pipes can be important but doing exclusively that on soap dm can make you worse at stickies (which are a lot more important) so I wouldn't recommend it all the time.
Termo's i52 povs give you a good idea of how to play and position yourself on demo but also are a good indicator of a good way to aim on demo (although it's not the only one) as his dm is really really good.
Practicing with a team is important as demo is hugely influenced by how people play around him and being consistently with the same players lets you adapt a good playstyle.
Think about ammo conservation, when to use each of your weapons, what targets you should be focusing, when to call focus etc. Try and mainly focus forwards at the chokes and don't focus on enemy players behind you when there's other players to deal with it and people to damage in front, you're the no 1 class who shouldn't get distracted. Think about whether you should be taking ubers or if your med can solo your pocket and you can stay out. Also think about how many heals/how much spam you want to be taking.
Remember, demo is a hard class and one that can't do much in dm situations a lot of the time so don't get frustrated when you get buttfucked by out of position players when you're reloading sometimes.

I'm already on a team, which is why i wanted to kick in gear some practice. I did highlander demo last season and i'm going into 6s demo with some of the members this season. I just meant in terms of 6s i've done lobs/pugs, we've got a scrim next week.

Also thanks for the advice. I'll probably go ahead and start checking out some POVs aswell as look back at my own games to check on what i can improve. thanks guys

Also if youre looking for resources, this list includes pretty much every guide there is on the internet about 6s, including lots on demoman