Everything should be made
as simple as possible, but no simpler.
-- various attributions
I start with Labyrinth Lord, except I use the 1st ed. AD&D/OSRIC hit dice for character class and weapon damage. Which character classes to use is still a fluid concept, including race-as-class. Thieves are still a conundrum, because I like Procedural Lockpicking and feel that many of the listed thieves' skills can be performed by anyone, although I'm intrigued byDyson Logos's 2d6 thief skills table.
Speaking of skills, I don't use them. Everything is based either on a character sheet ability, the resourcefulness of the players, common sense judgments, or a die roll as determined below.
AC ascends, and starts at 10. I use the Dex bonus to hit, and add the Str bonus to damage, for both missile and hand-to-hand combat. Intitiative is individual, roll 1d10 + Dex modifier.
Saving throws are mapped from Castles & Crusades, except the base number is determined by the saving throw in Swords & Wizardry. I add a modifier to the roll based on class, any attribute bonus, and selected prime attributes" (from C&C, but with different values), and use the C&C mapping of old-school saving throw types to new (e.g., "Breath Weapon" = Dex).
Anything not covered in the rules that can be resolved as a yes or no question, I use a d20 and a save. Anything where success can be measured in degrees, I use either a generic 2d6 table adapted from the Moldvay/LL "reaction roll" table, or 3d6 (or more) vs. some attribute. The latter is more likely to be used in contested challenges.
Magic use is per Holmes, plus a bit of 1st ed. Material Component fluff. I use the Holmes scroll creation rules so my low-level M-Us can have a little more firepower. I have two house-ruled exceptions to spell use: Read Magic and Detect Magic are both innate skills of Magic Users, although it takes 10+ minutes to use, whereas casting them as spells is near-instantaneous.
I have my own set of grappling rules. Combat maneuvers are like Telecanter's, except I use a saving throw to determine whether damage is taken or the maneuver succeeds. Languages are also per Telecanter, although I have a slightly more detailed way of handling them. Encumbrance is per Lamentations of the Flame Princess, which is also mentioned in the Telecanter document.
It's that simple.