the first step is the hardest part, the rest of the guide is easier from here.
Step 1 is to look at yourself, your playstyle, and your end-goal on booting up a multiplayer race.
a few of many different reasons can be :
-Multiplayer races for daily missions
-Farming for multiplayer pass rewards
-Wanting to race other players

and it can be any reason under the sun! but at the end of the day, you aren't there to ruin it for other people. you're there for you.

Once you know your reasoning for why you play, you can begin to analyze how you race already - most of you may not even need to follow this guide any farther than this, finding you race just fine!

- However, it's been shown to me there are those of you either willingly or unwillingly driving against other players as if they're Computer Characters.

"Courtesy - to show politeness in ones' attitude and behavior towards others"

Racing courteously is as simple as the thought - "Do I want this to happen to me?"
if the answer is no, then it's usually a very respectable idea to *not* do it.

Examples being :
-slamming into the side of another car on a straight just to screw their line up
-purposefully crashing into other players, effectively ruining the race for yourself as well as the target
-bullying racers into stage hazards/walls they would have otherwise been able to avoid

in most cases, even if you don't care for video-games at all, having things like the listed above happen to you just end up making it less fun for everyone involved. Best case, you shrug it off as a ruined race and continue - worst case it ruins someones day, trying to relax after work.

Being mindful of your actions in relation to others will not only improve the attitude of players surrounding you, but also improve your own strategy - being more aware of others means you can better predict and outplay.

With the meat of what I wanted to explain written out, I'll leave the end short and sweet. Thank you for reading, and good luck in the races!

TL:DR - don't play like someone you'd dislike playing with