Creating Monsters in AD&D 2nd Ed.

[Yora]

I want to create some custom monsters for 2nd Edition, but I'm not yet very familiar with and used to the system.
The DMG has a table to calculate XP for a monster and the MM mentions that THAC0 is always based on HD, yet doesn't say how. Are there any rough guidelines somewhere about how to make new monsters?

[Lord Torath]

The DMG has Monster THAC0s in Chapter 9 (pg 53 in the 1st printing).
XP calculation is on page 47 (Chapter 8).
Saving throws are generally the same as a fighter of the same level/HD, unless your monster is unintelligent, in which case all saves except for Poison, Paralyzation, and Death magic are as a fighter of half the monster's HD.
But you should feel free to use whichever table you feel is appropriate.

Other than that, you just step your way through the blank form at the back of the Monstrous Manual

[Matthew]

There is a table in the PHB for HD to THAC0. Basically it goes down two for every two hit dice. HD 1-2 = THAC0 19, HD 3-4 = THAC0 17 and so on. You basically shift experience value up a level for every significant advantage a monster has in second edition. It was a bit more granular in first edition.