Well for Fox v Marth it can be a bit difficult because you feel as if you're gonna get stuffed at whatever approach you make but a big tip in the match up is, don't approach too much unless you have a solid understanding of their movement and their attack patterns.
First tip would be for approaching I guess, A lot of times if you're fighting a good marth you'll see him dash dancing and waiting for an opening, this is where honestly I find the match up incredibly fun.
Marth is vulnerable on the turn around of his dash dance, what you can do is running shine (INCREDIBLY important in this match up) to break up his dash dance, although it isn't as easy at it sounds, you need to follow his movement and overshoot the shine because he's obviously going to be running from you.
Once you get an opening from that shine, the optimal thing is to waveshine after it and grab him ( practice this on 20XX or reg melee) then proceed to upthrow upair at low percents because you can combo him with up to 3 upairs. By then they will have easily gotten 60% on them and all you have to do is play patient and wait for a whiff and shoot a couple of lasers. Unsafe approaches against marth are what he preys upon so don't go committing to a dash attack or something and do not get impatient and whiff upsmashes, that's the worst thing you could do.
Now Im gonna simply guess here but you're probably facing the new nooby marths that just basically spam fair to wall out people without thinking. Do a lot of fsmashing on your approach and fairing towards you. At these lower levels there is a lovely tool you have called shield stops and shield DI.
Shield DI is when you angle your shield (aka move it) but the thing with run in and shield while angling it forward is that if you have a prediction on when you think they attack. Marth's moves (like approaching nair and fair) will have a lot of lag on shield and you can easily grab him from it or aerial out of shield if you get them to hit it with your shield approach. You of course will already know your OOS options as you're a big boy and I dont need to explain everything to you.
But this is probably the most important thing to do in most match ups and this match up, on stages like fd, and honestly most stages too marths will usually go for a grab after they see you whiff off of a dash dance, they have no self control dude they just want it badly. SO what you can do is fake a whiff with an (undershot move or aerial) then cover it with some other fast move or tilt, such as shine. Armada is incredible at this and you can watch his sets against m2k and PP which shows how he mixes up his approaches, covers his mistakes, and breaks up their dash dances.
Sorry for the big wall but yeah these are some tips I found useful when first starting to practice the match up. You have to be patient and wait for an opening really or use something low committal like running shine to break up their DD and make that opening.
Extra: You should of course look up kill percents and such as well as set ups on him, jab upsmash is pretty good once you understand his movement and can exploit it, once you reach a bit higher not so nooby spam level you'll probably find this match up super fun.
EXTRA EXTRA: Lasers and SHDL is not overrated at all, practice it and practice turning around in the air then shooting them, it's incredible useful against marth to apply pressure from afar to make him approach as well as racking up percent since he dies rather quickly.
