Mana Points
You don't have uses per day of any class feature, feat, spell, etc. that normally provided you with 2+ uses per day or required you to spend points such as arcane points, grit, ki points, bardic performance rounds per day, rage rounds per day, etc. Instead you get one big mana pool and spend mana points to do all those things. When you begin casting a spell or other effect that requires MP, you invest the MP and make all MP-related decisions at the start of casting and cannot change these decisions afterwards. Aborting or losing a spell or effect still causes the MP to be lost even if the effect isn't cast.
EFFECT TIERS
The term "spell" is replaced with "power" and "spell level" is replaced with "tier". All class features, feats, monster abilities, powers, racial traits, and traits can also have tiers. Feats and traits with no prerequisites are always tier 1 unless they scale with level (for example, granting you a bonus that stacks with anything else that grows with level).
DETERMINING MANA POOL
To determine how many mana points you gain at every level, subtract your hit die size (6, 8, 10, or 12) plus skill points (2, 4, 6, or 8) from 20. The remaining amount, plus your highest mental attribute modifier (Int, Wis, or Cha), is the amount of mana points you get per level.
For example, a Bard has a d6 hit die and 6 skill ranks per level. 20 - (6+6=12) = 8. So a bard gets a number of mana points at every level equal to 8 + highest mental attribute modifier.
MANA COSTS
*Bardic performance, rage, and similar effects that are used in round-by-round increments. For abilities that use other increments of time per day based on class level (such as many Alchemist Discoveries and Witch Hexes), treat them as growing class features with a duration per use equal to the base duration times the class level. For example, an alchemist's Wings discovery lets them have a fly speed for a number of minutes per day equal to class level that they can split up in 1 minute increments. This system would adjust the ability to cost 1 MP per alchemist level per use but always last up to 1 minute per alchemist level per use (making it both cheaper and longer lasting in general).
Features with set uses per day based on class level like Smite and Wild Shape are considered growing features and simply cost 1 MP/effective class level per use.
Other Features
Any other feat, feature, etc. that had limited uses per day but doesn't have a spell tier, is given an effective spell tier to calculate its cost. If it does NOT grow with class level, then its effective spell tier is half the class level at which it is gained (round down). If it DOES grow with level, then its effective spell tier is one-third your level (round up). Note that any ability that grants you an extra action, reroll, attack roll, saving throw, skill check, etc. are always growing with you because your bonuses to these rolls grow with you and your total level would be your class level for these things since they still increase with multiclassing.
Spontaneous
Spontaneous spells cost less mana to cast equal to half your spellcasting ability modifier (to a minimum cost of 1 mana). This doesn't apply to special class features that convert prepared spell slots to spontaneous spells.
Recovering Mana Points
After a short rest, you regain an amount of MP up to your level. After a long rest, you refill your entire MP pool.
Over Casting
If you have at least 1 MP left, you can use any effect that costs MP, regardless of cost. However, if you don't have enough MP to use the effect, you're reduced to 0 MP, lose HP equal to the difference (as if by lethal damage), and must make a Fortitude save against the effect's DC or become staggered for 1 round and start taking 1d6 + (effect's tier) bleed damage. You make a new Fortitude save against this effect every round, ending the bleed once you succeed.
Under Casting
You can halve the MP cost of activating an effect by tripling the casting time. This doesn't apply to effects that can be used as free actions or reactions. Once you decide to cast this way, you can't change the casting time and must finish casting or abort the effect. Aborting or losing the effect due to failed concentration causes you to lose the MP invested.