I don't have the playtest rules handy at the moment, but I wonder how many of the MM monsters might be quickly usable as PC races. In general, playing a monster as PC requires it to be able to gain XP and level up. In absence of rules for "levelling up in monster's levels", let's focus on using monsters with classes (just like races do).
1- If the monster doesn't have any ability score higher than 20, how about using its stats as-is, as starting PC stats? A gaming group who only uses point-buy is not going to like this, but a gaming group that normally uses rolled abilities should have less problem, as long as the stats are valid results. (Alternatively, extrapolate bonus/penalties by comparing stats with 10, but this is more complicated).
2- Ignore the monster's hit points, use HP/HD normally by your class and level.
3- Ignore the monster's weapon attacks numbers, use proficiencies by your class and level, but add natural attacks to your proficiencies. Maybe also add signature weapons.
4- For skills, look carefully if some of the listed ones have clearly high bonuses, and add them to your proficiencies.
5- Use all monster's special abilities, and if they have a DC, calculate that as per spellcasters' spells DC, with proficiency bonus by your character level.
Unfortunately I can't make a concrete example right now, but it might go like this:
Schlumpf, as MM monster
Str 8, Dex 13, Con 10, Int 12, Wis 15, Cha 11
12 HP
Attacks: shortsword -1 (1d6 dmg), shortbow +1 (1d6 dmg)
Skills: Perception +2, Stealth +5
Special: Charm Person 1/day DC 15, can hide in dim light
Schlumpf, as race
Str 8, Dex 13, Con 10, Int 12, Wis 15, Cha 11 (fixed starting array)
(No additional weapon proficiencies)
Skills: proficiency in Stealth
Special: Charm Person 1/day DC=8+Cha+prof, can hide in dim light