Write down HD. That’s the one stat you need to have pre-committed.
And roll that many 1d8 for HP later when you need it.
For AC, use half HD + 12.
For attack bonus, use half HD + 2.
For bad stats, checks and saves, use +0, for good stats, checks, and saves, use half HD +2.
For number of attacks, use half HD but at least one.
Each attack deals 1d6 damage.
For CR so you can look up XP value or appropriate treasure: 1HD → CR0, 2HD → CR⅛, 3HD→CR¼, 5HD→CR½, 7HD→CR1, each 3HD over that adds a CR.
You can easily override any of these by writing it down. The easiest and best thing to override is the attack line. Write the damage expressions separated by slashes.
For example, a “HD2 1d4/1d4/1d3” creature has three attacks, two that deal 1d4 and one that deals 1d3.
A “HD4 1d6/1d4 + con save or 2d6 more” creature has two attacks, one that deals 1d6, and another that deals 1d4 base but 2d6 more unless the victim can make a constitution saving throw.
Similarly, “AC18 HD2” creature is unusually hardy (but attacks with a normal +4 1d6 attack). Or w/e. Or it can be a kind-hearted creature like “HD1, no attacks”. That way you still have a way to calculate AC or saves for it incase it gets in the crossfire of a fireball or whatever.
Even if you use gold for XP, figuring out how dangerous the monster is CR-wise helps you calculate the appropriate amount of treasure for it. This is instead of Treasure Type as per Moldvay.
This isn’t the only or even best way to do monster stats. It’s not even the method I use most. Usually I base custom monsters off of a preexisting monster and change details. But… in my philosophy, everything needs stats. If it exists in the world, it needs stats. And like the least you can do when prepping is put in one number for each creature in the text. This is great when you are converting from other game systems or even from novels and comics.
My aim is to prep very widely. Maybe the player characters will never ever reach this particular island or castle. I don’t want to put too much effort in it until I suspect they are going to go there. (If you have a strong idea of where the characters are going, it’s good to carefully read up and study that area. But if they are going all over the place and keep surprising you, well, they’ll have to stand some “DM loading times” as you look at your “uh, this creature is a… three?” notes.)
Formula for the HD↔CR stuff for HD lower than seven (or for CR lower than 1): The monster gets 1HD for free. Each HD after that costs ⅛ CR. Mnemonic: “The first one is always free, then you can buy one more for [pirate parrot voice] pieces of eight! pieces of eight!”.
Formula for the HD↔CR stuff for HD higher than seven (or for CR 1 or higher): The monster got the first four HD for free. Every three HD after that costs 1CR. Mnemonic: “CR was originally based around a 4 character party of the same level. Historically, each party member could handle a HD each, and everyone except the mage could handle one extra HD per level. The mage died.”
That last mnemonic is not to say that this is a robust encounter balancing mechanic by any means. Balance areas against other areas so you get a gradient of difficulty. HD3 creatures are tougher than the same amount of HD1 creatures, that’s a no-brainer. But it’s impossible in 5e to balance encounters to the party. And you don’t always have four players. It was just meant as a fun way to remember the CR formula.
The CR→XP formula on the other hand, wow… that’s rough to memorize. But for CR 1 or lower, every ⅛ is 25. So CR 1 is 200.
The reason it’s hard to memorize is that it jumps all over the place with level tiers. For the first three levels, a four-member party defeating one monster of their own CR gains around a sixth of the XP for next level. Then at level 3, they gain around a tenth of the XP to get to level 4. Then from levels four to 10 they gain around one-fifteenth (so they need to face 15 encounters per level). Then from level 11 on it’s back to one-tenth.
This sorta matches with the normal level tiers, which are 1-4, 5-10, 11-16, 17-20. But here it’s 1-3, 4-10, 11-20.
Or ya can just do the lazy and multiply the HP you rolled with 23 to get the XP. It’s very rough but at least it gives you a number.