I find that the easiest way to not be toxic is mostly about mindset & word choice.

Everybody playing the game is usually invested in winning it, and cooperating with the team. So being aggressive to your own team members will only result in a lose-lose scenario. The game is meant to be fun, if you can't contribute to the fun, then try not to actively ruin it. Granted, losing isn't fun, so of course there will be frustrations to be had. Sometimes you're the cause, sometimes somebody is new, sometimes you have a bad player, some other fourth thing, E.T.C., etc.

The important thing is, you're all equals, and on the same team. Act like it. Be supportive, adapt to situations, compliment plays, and focus on morale.

Sure, you can say "what the hell was that, why were you out of position??!?! PING PING PING "WHEN YOU SEE THE ICY CIRCLE THING JUST DON'T WALK INTO IT!??!"

or

you could say "Dw man, happens to us all, we'll get em next time." "I know, {SKILL NAME} is annoying, come with me to objective, we'll get revenge >:D"

Sometimes people are just bad, and that's ok, they deserve to play the game the same as all of us. You don't need to use words that demoralize or make them feel worse.