Last Will must be activated before a monster is sent to your Graveyard. Additionally, when a monster is sent to your graveyard, you must immediately decide to activate the special summoning effect of Last Will.
You can apply the effect of “Last Will” as long as a monster on your side of the field is (or was) sent to your Graveyard, no matter which Phase it was in. As long as it’s the same turn in which you activated “Last Will”.
If you activate 2 "Last Wills", then you get to Special Summon 2 monsters, as long as a monster is (or was) sent from your side of the field to your Graveyard that same turn.
You can Special Summon for “Last Will” if a monster was Tributed for a Tribute Summon, or for "Cannon Soldier’s" effect, etc.
You can use "Last Will’s" effect to Special Summon a Level 5 or higher monster like "Labyrinth Wall".
You cannot Special Summon a "Special Summon-only" monster. You CAN Special Summon "Toon Mermaid" with "Last Will's" effect, as long as you have "Toon World" on your side of the field.
"Last Will’s" effect can only be used if your OWN monster is (or was) sent from YOUR side of the field to YOUR Graveyard.
If your monster is sent to your Graveyard by battle, you resolve "Last Will’s" effect during that same Battle Phase (but not during the Damage Step). If it’s your turn, then your newly Summoned monster can attack before Main Phase 2.
If "Imperial Order" is activated after "Last Will" has resolved, it will not negate "Last Will's" effect.
You can Special Summon a 1500 ATK monster that will have higher than 1500 ATK after being Summoned, such as a Fiend with 1500 original ATK while "Yami" is on the field (its ATK once Summoned will be 1700). You can Special Summon "Muka Muka" even if you have 6 cards in your hand and "Muka Muka's" ATK, once Summoned, will be higher than 1500.