As a spy, you're best off spending your time working to assassinate the opposing medic. Since the other team has only one medic, it is a huge advantage to your team if you can kill (or badly hurt) the other medic, even at the expense of your own life.
Since this is a competition-level play, you can expect full communication from the other team. You will very rarely fool them into thinking you're on their team. They all should know where everybody is. (Whether or not they really do is another matter). The simplest way for you to be effective is to find a nice sport to Cloak and Dagger, wait for the enemy medic to run by, and revolver or ambassador them from a distance.
Mix up your spots, be unpredictable, and stay hidden. You may also want to spend some time hounding the enemy sniper, depending on his skill level. I've seen highlander teams completely fall apart from a skilled enemy sniper. On the off chance he's razorbacking, the ambassador will be the weapon you want to go with. Mix your disguises, but definitely use the sniper disguise - that class matches closest with this play-style.
Yukon was intentionally designed to be harder for Engineers:
This abandoned mountaintop coalmine was repurposed by shadowy operatives into a top secret surveillance outpost—one which also, so nobody gets wise, insidiously continues to mine coal. There is also a log hanging in the center of the map—possibly as an example to other logs. Logs that said too much.
Five control points promise a lot of momentum swings in this compact outdoor map, with plenty of tunnels and rocky outcroppings. Plenty of alternate routes ensure that if you can’t get past a well-placed engineer through one tunnel, you might be able to sneak up behind them through another.
CP_Yukon is a community map from the brilliant dual-powered brain trust of Patrick “MangyCarface” Mulholland and Joachim “Acegikmo” Holmér.
-- Classless Update, Day 3
Point 1
Point 1 is extremely poor for defending as Engineer.
If you're building full size sentries, it may be better to build them down in the pit, although that means a Spy can back-cap the point.
Point 2
Point 2 has a few defense spots. One of the best for actually covering the point is at your team's end of the hall when you first come out the door from point 1. You had better have the Wrangler, though, or a sniper at the far end of the can easily take you out.
Another point is past point 2, on the middle route. You can place a sentry on the walkway up. The problem is that, if you place is close enough to the point to be able to see it, it's far enough from the point 3 area for Demomen to be able to take it out.
Point 3
Thanks to point 3 being moved, your best bet is near the point itself, preferably where the fence covers it, so it can't be shot by Snipers (or at least where you can't be shot by Snipers). Your biggest problem here is people sneaking up behind you. You could place it at the very end of the point 3 bridge, facing the point to help counteract that.
Point 4
Building slightly to the enemy's side of point 4 works pretty well, as it covers both entrances to the point 4 area.
Another Alternative
Due to the difficultly in guarding points with a level 3 gun on this map, Mini-sentries work fairly well on it, at least when combined with the Frontier Justice.
Best Answer
Always face towards and in the direction you wish to move (across the ramp.) Move left and right (strafing) and avoid pressing the forward or backward keys as they will often cause you to stop. Jumping or crouching is often not necessary.