First you'd want to set something up to detect any key presses, this can be done by setting macros for those keys, in BYOND 511+ you can use the Any macro.

Next, you'd store a list of key presses as they're triggered, occasionally trimming the first elements of the list away to break slow combos.

Then you'd want to check the list of stored key presses against your valid combos and execute whatever action you want those combos to entail when matches are found.