The DMG advises against allowing players to begin the game as monsters, and Demon Idol agrees. However, a player might end up with a monster character via magic such as reincarnation.

Scores should be rolled randomly with whatever method (I-IV) player desires. Modifiers and mins/maxes should be customized by the DM to suit the form. The DM should come up with these for the form, preferably with the agreement of the first player to have a character of that form.

As an example, here are modifiers and min/max for a centaur form:

Charisma penalties are removed when dealing with their own race (just like dwarves and half-orcs).

These also need to be determined by the DM. Consider the monster, its relation to the setting, and what makes most sense. It can be good to give every monster at least one class in which they are unlimited in advancement. However, in the end, level caps depend on the setting, the game, whether UA caps are being used¹⁾, and what kind of game the DM prefers.

Choose a race that best matches the monster's form. If no mention of longevity is made in the monster's description, just use the human age brackets (and starting ages, in the case of reincarnation).