Super basic, there's two parts to the weapon: The glaive and the bug. Note that I'm playing on PS4, so all button names are PS4, just transpose if you're on Xbox.
The Glaive:
This is the big stick that you use to hit things until they be dead. This thing's all pretty self-explanatory. The one thing to keep in mind that while you can hit RT+X to launch yourself up in the air and then use that to try and mount the monster, doing this all the time won't work. Like resistances, each time you successfully mount the monster, the next "mountable" damage threshold goes up, which means you'll have to apply more damage before the mount will actually stick. So don't bother spamming the jump and wonder why you're not mounting, and maybe try and save the mounts for when you'd really like a knockdown.
The Bug:
This is where the glaive gets most of its complexity from. The bug sits on your arm, and during combat, you can send it flying out in front of you to hit things. The controls are this:
LT - Holding this puts you in "bug mode". You can still run around in bug mode, but the rest of the moves are bug moves.
LT+Triangle - Send the bug flying straight out ahead. This is based on your character facing, not screen orientation. It keeps the same horizonal position, so if you fire it at a monster up slope, it'll hit the ground. This is frustrating and aiming the thing right is a definite skill hurdle. It'll keep flying out until either you let go of LT, call it back, or it hits its distance limit. As you level up the bug, the max distance can change.
LT+Circle - Call the bug back. It'll turn around, come back, and when it gets to you, will give you whatever juice it's carrying (more on this in a minute).
LT+RT - Shoot out a bug marker at the targeting reticle in the center of the screen. If you do this and then send out the bug, it'll home in on the marker instead of flying straight out. This can be handy to hit a particular part of a monster, but it's not super reliable. First, you have to hit the right spot with the marker. Second, bug will go straight at the marker, even if something else is in the way, so it might hit terrain or a different part of the monster. Basically, use this if you want, but most people just manually aim the bug.
Juices:
This is the real heart of the glaive. The meter we're interested in is the one just below the sharpness meter, seen here:
Whenever the bug hits a monster, it extracts juice from it in a certain color, depending on where you hit it. The left-hand box on that meter will show you what color juice the bug is holding. If the bug hits a monster again while it's holding juice, it'll swap to the new color, because it can only hold one type of juice at once. When you call the bug back while it's holding juice, it'll deposit the juice in one of the other three slots in the meter.
So why the heck do you want juice? BUFFS! These are the four juice colors, what they do, and where they come from. Note that the "where" is general, and differs from monster to monster. You'll need to experiment a bit:
Red - Increases the damage you do. Usually comes from a monster's head.
Yellow - Increases your defense. Usually comes from a monster's body.
White - Increases your speed. Usually comes from a monster's legs and wings.
Green - Heals you. Usually comes from a monster's tail.
All big monsters will always give you red, yellow, and white. Green isn't always given, so don't kill yourself looking for the magic spot. Small monsters will only give a single color.
When you get green juice, it's used instantly, and doesn't go in the meter. The other three colors each have a designated spot in the three diamonds of the meter, and will give you their buff as long as they're there (the meter has a timer that will empty the meter when it times out). Your main goal in life is to
keep all three colors of juice in that meter. When you get all three colors in the meter, you go into your SUPER BUG MAN state. You'll know you're there because your stick gets a cool red glowing trail when you swing it. When you're Super Bug Man, all your attacks become much faster, and some combos will even get extra hits automatically added to the animation. This is on top of the stats buffs. It's generally a really good thing, so make it your goal in life to keep that meter full.
The real trick of the glaive is knowing where to hit the monster to get each color of juices, being able to do that quickly and consistently, and being able to balance that with dodging and attacking while gathering juice. It's a really fun balancing act once you wrap your head around what you should be doing and how to do it.
One thing to keep in mind with the glaive is that some of the attack animations are pretty long. It's really tempting to get in there and just cut loose with a long combo, since it's easy and looks sweet. The problem is that if you suddenly need to dodge, you can wind up getting stuck swinging your stick around instead of dodging. Resist the urge to always go long combos, be deliberate, and use your mobility to stick and move. Save long combos for when you knock the monster down with a mount.
Outside of hunts, there's some extra complexity with upgrading your bug. Like weapons, bugs can have damage types (slash vs blunt), and different elemental and status effects. This time around, your bug and stick can be mixed and matched, and they need to be upgraded independently. I haven't dived deep enough into this to give you a rundown, so check some upgrade guides when you get that far. Hopefully this helps!