Rocket League

EASIEST WAY!
-1v1
-All-Star Bot difficulty
-Max 1 Goal
Equip Hearts Decal (Or any decal apparently)
(It has been patched, people are reporting unlocking this with any decal as of November 2023)

As soon as you start the match, move out of the way, when the AI hits the ball and its about to go into the goal, pause the game, switch to the opposite team, as soon as the ball goes in, you will get the achievement for winning.

Clearly the down votes are because they do not like earning achievements.

EASIEST WAY!

The first thing you need for this achievement is the Hearts Decal pattern. This will be one of the many rewards for completing a match. You can quickly collect items by starting an Exhibition match with no bots and set scoring limit to 1. Not all designs are available for every car. At this time, only Zippy and Scarab can get the Hearts decal.

Now that you have hearts equipped from the garage, start an Exhibition match with these settings.

-2v2
-All-Star Bot difficulty

Mutator settings should be default with this change:
-Score maximum 1

Using 2v2 you have a teammate on the same level as the opponents without making it too difficult to score. If the other team scores simply restart and try again. Just play smart and try to take advantage of shots near the goal. Be ready to play defensive even though your teammate does a good job of it. With some luck, your teammate may even make the goal for you which will still unlock the achievement. Good luck, and I hope this helps.

NOTE: If you meet the requirements and this doesn't unlock double check that you are connected to the Rocket League servers. If you're not, then try to get them back on by starting an online match then try again.

Before you can earn this achievement you must unlock the hearts decal. This is typically random and you'll unlock it for a number of cars as you play the game. Either play loads of short exhibition games until it unlocks or be patient, it'll come.

Once you have the hearts decal, find the car which it applies to and apply it to that car. Leave the garage.

You then need to set up an exhibition game against the AI. Go to Exhibition and select any arena you like. It's useful for the game to be either 1v1 or 2v2 as it's less chaotic and gives you more chances to score. Don't forget to set their difficulty to All-Star.

It's also helpful to set mutators. The ones which I would recommend would be the goal limit being set to 1 which would ensure the game remains short and can be restarted upon failure. The second mutator I would recommend is the unlimited boost as this will allow you to quickly hit your opponent on the attack or get back to defend, if necessary.

Once these settings have been configured, head into the game. I would recommend you play on the defensive until you see and opening and try to score. If you do, the game should end and the achievement should pop. If they score, restart the game and try again.

For a detailed explanation and demonstration check out this video.

This solution may help anyone having trouble unlocking this as I did. After winning several matches against All Star Bots with hearts decal equipped I gave up thinking this achievement was glitched. Then I tried one last thing, I equipped Hearts decal and ALSO the hearts antenna, won a match against All Star bots and BOOM the achievement popped Good luck

Equip the hearts decal and press play now then hit custom games exabation and then set bot difficulty all star and max score 1 goal in a 1v1 game.

In the match have the oponent kick the ball to your side of the stadium and pause the game and select change team change your team to the oppsing team and it will say that your team scored.

Go to manage inventory see what car has the hearts decal then equip it on that car then go to local and exhibition
bots-all-star
1v1 (doesnt matter)
mutator settings and make max goals 1 so you dont have to play for 5 mins and just score a goal match over cheevo pop

I figured this out from the comments on some of the other solutions, but I wanted to get this front and center as the game has changed a lot since launch and many of the solutions aren't quite accurate.

You no longer need the Hearts decal, any decal will do the trick.
As far as getting the win, the other solutions have some helpful tips, but all-star bots you should be able to limit it to a 1 goal win and just get a cheap goal at the very beginning by either:
*If you are at an angle to the goal: just boost and accelerate straight at it, and try to hit it with the corner of your bumper so it gets knocked in at an angle
*If you are straight on: accelerating and boosting straight at the ball and braking at the last moment. (If you don't brake, it will likely go too high and bounce off the top of the goal). You can also hop at the last second instead of braking, to lower the trajectory.