Overwatch 2

As Symettra place 6 of your turrents in a room and wait for an enemy to come in and get trapped. The 7th beam is your gun and shoot at them.

Like other solutions have mentioned, place 6 turrets and use your own weapon (beam or ball). This solution is just a list of maps that have the easiest rooms to set up your car wash and some general tips.

Temple of Anubis - Defence.
From point A looking toward the enemy spawn there is a gate they have to pass through. To the right there's a small room which allows them to flank, but inside there there's room for you and your turrets.

Hanamura - Defence.
Here there's also a gate at point A where all players that can't climb/fly have to go through. Before the gate on the left there's a room where enemies usually only enter from one direction.

Volskaya Industries - Defence.
Once again at point A there's a form of gate, with a truck in the middle. To the left and the right of the gateway there are rooms.

Numbani - Defence.
At the first objective there is a hallway to the right (bottom floor) with a larger room with a health pack inside and at the end an entrance where the enemies are very likely to come through. Try to place them very high up so people won't notice them.

Watchpoint: Gibraltar - Defence.
At the first checkpoint there's a room on the side which the enemy often uses to flank.
A bit after the second checkpoint there's 2 doors next to the big door the objective comes through. Keep in mind that the objective door is usually the main way for attackers to go into that area, but it closes as the objective moves past it.

General Tips
- Try to do this at the start of the match, because it takes some time to place 6 turrets.
- Try to place turrets on walls away from the floor, because Junkrat's grenades or Phara's rockets could destroy them.
- Try to bait people into the room you're set up by using your (default) attack. Since it travels slowly and enemies see where it's coming from.
- Try not to bait a Winston, since he wrecks turrets and requires little aim effort to do so

As of 2020, this achievement must of changed. Looking at the other guides, everyone is saying you need 7 beams, but now it is only 3. Symmetra must of got nerfed at some point because she can now only have 3 turrets placed at a time. At max, Symmetra is able to hit a player with 4 beams, but the achievement only requires 3. This being said, having your Photon Projector (her weapon) along with 2 turrets hit a single enemy at the same time will unlock this achievement. Also, just placing all three of your turrets and they all hit the same enemy the achievement will also unlock. As said in the other solutions, getting a single enemy to follow you into a room is the best way to do this, even if it is easier said than done.

* I got this achievement by using 2 turrets and my weapon

Note: After this change, I feel like this achievement got significantly easier

Overwatch 2 Solution

Quick Solution

Place 3 turrets at a doorway and bait someone to follow you. You can also place these turrets in areas that are commonly walked through.

What do Sentry Turrets do?

Sentry Turrets are bound to Symmetra's Ability 1 which is by default
When the ability used, Symmetra will throw a turret in a straight line and, when it hits a wall, it will attach itself to said wall. Symmetra can hold 3 turrets at a time and she can only place 3 turrets at a time. When a fourth turret is placed, the oldest turret will be destroyed.
Symmetra's Sentry Turrets will do little damage to the enemy, but will slow them down once they get close enough (this stacks).

How do I unlock Car Wash?

I would suggest going for this achievement on Defense as you will have your chance to set up turrets. Set up your turrets at a choke or small doorway. You can even set them up on point as your enemies are trying to reach it.

Alternatively, you can place all your turrets together and then use your teleporter near an enemy to instantly transport the turrets near the enemy as seen below.

How do I boost this achievement?

The Lockdown Method

This is the most common method to boost Overwatch that requires 10 people.

1. Get a group of 10 people.
2. Get into two groups of 5
3. Disable cross play
4. Queue for Quick Play Open Queue at the same time and hope it matches you guys up
5. Once you match up, set up a room with all 3 turrets and then have someone from the opposing team walk into the room.

Remember to keep the objective contested unless it's a Control map which will not need contesting unless someone captured it.

Here’s what I did:

1) I unlocked this on Watchpoint: Gibraltar in defender Escort mode (as we do with most Achievements). The payload is in a nice, contained area at the beginning and end of the fight; it seems to take forever.
2) Symmetra begins a match with the ability to place three turrets with her LB ability. Over time, her cool down will allow for a fourth, then fifth, and so on. She’ll only be able to place six turrets at one time… so the seventh one you try to place will replace your first.
3) Try to put all of your turrets in a grouped, yet hidden space. I placed mine on the opposite side of a doorway, so incoming enemies wouldn’t see them until its too late.
4) Once you’ve placed your turrets, it’s a game of luck — a lucky rocket could take them all out super quickly. Play the game as normal, assisting teammates and going for additional kills. Hope for the best.
5) When your beams target someone, use your pistol as the seventh beam.

The Achievement will unlock when an enemy is simply hit by all six turrets and your weapon. Kills aren’t important, but they are better, of course.