Just a heads up, It's been a week since we got started and everyone save one player has a character ready, it looks like. I don't think it will take too much of a feat of mental gymnastics to incorporate the final PC into the party once they are done with it, so I'm thinking that we'll start the game proper tomorrow.
A few things I can think of off the top of my head, which might help once we begin.
1. Unlike 5e, where everyone makes a single Initiative roll which lasts the entire encounter, in B/X, players, NPCs, and Monsters re-roll Initiative at the beginning of every round. I find it's a lot faster if I just make the rolls for each PC rather than taking up one or teo days waiting for Initiative to get sorted out every round. So usually at the end of a round, I make a roll for everyone and post the turn order.
2. When in combat, I highly recommend you wait to post your character's actions
on their turn. A lot of players have a tendency to just all post what they are doing right at the start of a new round, and when their PCs turn comes around in the turn order, whatever they declared for their PC is obsolete because their target isn't in the space they were in at the start of the round, or their target has been killed by another PC already, or whatever. Then we get held up with the DM telling the player they can't do what they said they were going to do at the beginning of the round, we all wait a day for the Player to retcon their post and say something else, dice rolls have to be resolved, and the whole game is bogged down with players retconning obsolete declarations.
So usually I say "Round 1 ends, and Round 2 begins!" You generally if you post "my PC does moves her and does this", I hold you to it. If you declared for your PC out of turn and that movement/action option is no longer feasible when their turn in the Initiative rolls around, they forfeit their action for that round. So I strongly recommend waiting until your PC's turn is actually up to post.
3.I generally use theater of the mind for roleplaying purposes, but for combat we'll use a battlemap. Generally a battlemap looks like this:
You'll notice the map has grid coordinates. Generally, when you move, you say, "I move my PC to M13 and fire my bow at the goblin on J15", or something to that effect. I move your tokens around the board as we go, and at the start of every new round I roll new initiatives for everyone and post an updated version of the battlemap. If at any point between the the beginning of each round you would like to see the most current version of the battlemap, just request to see the updated map and I'll post a screenshot of it.
I believe most characters have a move of 120'. That translates to 40' per turn. On your turn you can move and then make an action, make an action then move, or move some then interrupt your movement with an action and then finish moving. However, if you move some, then make an action then finish moving, you only get half of your remaining movement rounded down. For example, if you have a movement of 40', and you move 20' to an goblin, hack at him with your sword, and then retreat again, you'll only get to retreat 10' (you move'd 20', had 20'feet remaining, but only get half of that after you make an action.
There are no "attack of opportunities (AOCs)" in B/X, so you can move up to an enemy, make an attack, and retreat again without worrying about AOCs, but be aware that you only get half your remaining movement to retreat, so you do put yourself at risk of your victim closing the distance on their turn and attacking you.
4. Most squares on the battlemap will be 5', unless posted otherwise. I count diagonal movement as 5' as well. I know that's not 100% accurate, but it's less complicated.
5.
DYING!!! If you're reduced to 0 Hit Points or less, you make 1 Death Save roll only on your PC's next turn. If they pass the Death Save roll, they are unconcious with 0 HP, and have to be healed or heal naturally through rest. Once they have 1HP again, they are conscious. If you fail your Death Save roll, you're just straight up dead.
I'm sure there will be more stuff but that's all I can think of off the top of my head at the moment.