I've written a bit of advice about being a good player, mostly for GURPS.
Trying to be a good player is hard. So let's be bad players! It's way easier. Or is it? Here is some advice to get the job done really poorly.
Nothing is harder for everyone else than running their characters. So, help out.
Remember to make sure to do the math for everyone else. Especially if they've done it for themselves. Re-calculate it. Check to make sure, with them and with the table in general, about what modifiers apply.
Tactically, they're probably a mess. Make sure you take time to give them advice on every aspect of their play - where to step, where to look, what weapons to use, everything. Oh, and don't neglect your guy - point out where they should go to help you do your thing. Not that you're not going to decide that later, anyway - but don't let them cut off your options.
Right after you give this advice, you want to make sure the person doesn't feel too pressured. A hearty, "nevermind, you go ahead, I was just going to say you could (fill in whatever Maneuver or action here), but you just do what you're going to do" is a good way to go. It's simple, clear, concise, and above all makes sure you put plenty of doubt in the person's head. That way, they're likely to listen to your help. Remember, it's their characters you need to mind.
Don't remember what you did last turn.
Or the one before, frankly. Stunned last turn? Forget it. Dropped your weapon? Swing with it anyway. Concentrate on the battlefield as a whole, and who should go where, and all the people you're helping out. The GM will probably remember for you. If not, how important could it be?
And don't bother looking up the effects of any spells on you, hostile or negative conditions, or debiliating effects of terrain you're on between when it happens and when you next go. Pleny of time to do that on your turn itself. Once the game can't proceed without your input, then find out what effects that all has on your paper man.
No need to know the rules.
"Rules mastery" is a new school concept for people who play wrong. Mastery of the rules shows you are a new-school noob who doesn't understand how it was done back in the day when Gygax and Arneson strode the earth, reffing dungeons from behind towering screens and rolling the dice for you. Or an old-school fool who doesn't get that you are Role-playing here, not Roll-playing. The GM will handle all of the rules. If you learn any of them, don't bother learning the ones that apply to your character and your situation.
Don't visualize your situation, either.
Also, only the rules matter. Unless they say X or Y specifically, then X or Y doesn't happen. The rules don't say you can't open a door with your shield hand while taking a Wait action with your sword ready while backing up to make space and stepping into the room if that's better . . . so clearly you can do it. Seeing the situation with your mind's eye and playing it out like a real person in that situation is very limiting. Don't limit yourself!
"Just make sure" of every single modifier.
For want of a nail, a kingdom was lost - remember that! For want of a -1 or a +1 on any given turn, an entire campaign can collapse. Better check on all of them. Check every time. Make sure if you have a conditional modifier to "just make sure" if it counts or not. Have a +1 versus demons? That orc has a pretty demonic-looking shield, so that's probably worth a +1. Is that a +1 to your skill and your roll against that skill? Probably. Better check. Have a +3 against poison? If you get poisoned, you better check and make sure it counts. It might or might not, so ask every time!
Alternatively, don't make sure of any of them, ever. You know your rolls. Just roll them. Circumstances may change, but negative modifiers probably even out in the wash. So just skip them all. You probably are missing some bonuses - make sure of them. Negative modifiers are the GM's job, anyway. The GM will tell you them, and if you've been told them before but not recently, like, on this turn, they're almost certainly gone. This is especially true of Disadvantages - they probably don't apply here, so don't fret them. The GM will let you know when they should apply. Playing out your disads in combat limits your choices . . . and that's the GM's job, not yours!
Don't plan ahead.
Figure out what to do on your turn. Knowing ahead of time is a sucker's game. Wait until you go, and then fully assess the circumstances. Make sure to get everyone's opinion on what you should do, and give yours on what they should do on their turn. Once they've established their plan, you can start on yours. After all, this turn's action has to be the most efficient, most tactically effective thing possible to do. Anything less is bad play, at best, and leads to a TPK just like all roads lead to Rome.
If you're using an unusual attack or spell, use it first, then figure out what it does. Not right away - wait until you absoutely need to know. If the enemy defends, no point figuring out how much damage you do with a headbutt, or the defense penalty against your attack, or the default roll for your technique. Oh sure, you probably need to know the last before you roll to hit, but that's not critical. GMs know this stuff of the top of their head. Besides, if you decide ahead of time and figure it out, the modifiers might have changed. This is especially true if you've been planning this move for a while! Pre-calculation, like rules mastery, takes away from the game. Don't do it.
Coming sometime - Part II - How to be a Bad Player - Non-Combat Edition.
Am I kidding? What do you think, am I? Don't decide now, wait until it's your turn and then start to think about it. We'll wait.