http://tf2maps.net/threads/valerias-guide-to-competitive-mapping.21718/

Read these.

Think about the areas you're making, and consider them from both perspectives (Offense and Defense) and find a balance where people on either side will enjoy the area they are playing in.
No one likes to be in a dead area that leaves them at a disadvantage (whether attacking or defending) so build areas that are neutrally useful.

Make the snipers work fucking hard to get picks

As someone who has tried to make 6v6 levels in the past (koth_satori, cp_banana) you need to have quite the knowledge and experience in hammer and tf2 level design to even get something half decent made. I made cp_banana 1.5 years into making levels, and it was still terrible. While I thought it may have been good, everything about it still had design issues that, while fine in a pub environment became major issues in a comp environment.

So honestly, truthful answer - Don't make a comp map as your first level. I know that's what you don't want to hear but everyone whose map has been played in even UGC has quite a few normal maps under their belt. Come over to TF2maps.net and get comfy in our community, you'll need the support.

People seem to underappreciate how much dedication and time it takes to be a level designer, especially for such a complex and deep mode as 6v6 tf2. Phi, Bakscratch and Scorpio have been at it for quite a while, some even from 2007.

Edit: I realise this makes me sound like an elitist asshole.. that's not how i envisioned this post.

Add a spire definitely

second CP needs to be not hard to defend against mid if you have uber, but hard to defend against mid if you don't. spires are an easy way to make it hard to defend without an uber without making defending with an uber harder

if you have more than three routes between last and second, it's going to be impossible to make sure nobody's hiding in them when you push out of last, so don't do that

elaborating on the previous statement, if you have a ton of corners in all the rooms between last and second, that will dramatically slow down people who are checking for lurkers and pushing out of last will be slow

there needs to be a demo rollout with at least a lunchbox on the way to mid, add a pill if there's serious risk of taking fall damage

mid needs to be less detailed than other areas of the map because it will have 12 players on it at once at the beginning of every single game, and people will rail all on your map with no compassion if they lose the map after taking framedrops at the midfight

valk's guide is pretty good for more concrete advice.

I disagree. He's not looking to map for money I assume, so without having any experience myself, I'd assume that trying to build a comp map is a great excercise in level design and a worthwile experience. And even if you fail at making a good map, you probably learn much more this way because you can really analyze what the design flaws are.
But I guess it depends whether u are more interested in the puzzle of designing the map in the first place or getting a finished product fast.

Learn how to seal different areas so that your map isnt a mess and people get over 12 fps
Just open up a decompiled tf2 map in hammer and see how cleverly each area of the map (2nd, mid, last) is sealed from the area after and before it.

Look how skybox brushes are being used and how nodraw seals the parts under the map so that there aren't any leaks and there is good area portal flow.
For mirroring 5cp/koth maps I've found my own tecnique. I create a visgroup with unique entities (light_environment, fog controller, logic_auto) and hide them when I want to do the mirroring. Next go to edit>select all and do paste special to duplicate every single object selected and create a visgroup with all the duplicated brushes.

Honestly, 5cp is probably the hardest mode to make maps for since mirroring and sealing are a pain in the ass and its hard to be original and appeal to a lot of players without them throwing shit in your face lol

make sure you dont have random shit blocking movement. Avoid tiny clippings coming out of the wall like they do in snakewater saw, and don't have random ceiling lights in tight corridors like badlands last.

Try to open up tiny indoor places (ie try to make granary 2nd or badlands last feel more like process lobby/IT). I think logjam does a good job of that if you look at the last to 2nd lobby.

listen to feedback

please

height difference - try to stay away from very large, inaccessible height difference or else non-soldiers feel useless, like on gran mid (which is a shit map)

don't overdo health/ammo

3 solid options to push into last and 1 shitty but sneaky option

2 ways to push from mid to 2nd and 1 shitty but sneaky option

don't make overpowered sniper sightlines, esp good sightlines from spawn doors

ramps make for fun jumping

spires make for fun airshotting

and please please please do that thing where you make stairs not eat rockets

EDIT: also most 6s maps are smaller than pub 5cp maps like foundry

please make terrain abuse-able or scouts are going to cry
i know i would

try to make cool sticky trap options but not ones that are hard to clear just maybe hard to see. also try to make the map balenced for all the classes, i love viaduct but it still has its flaws in the balance department. you also want to make each of the pushing spots have their own flaws and benefits like how pushing in from IT on process if good because its closer the the the exits for the other team but its way easier to get forced, Variety is always apreciated.

http://www.teamfortress.tv/28714/lets-try-to-point-out-what-makes-a-good-6s-map

Aaand something inhttp://www.teamfortress.tv/28646/we-need-to-encourage-tf2-map-makers