If I play Baited Hook to kill a Herald, first I must resolve the effect of Baited Hook and after resolve the effect of Deathknell?

Actually, you do not choose the order. The resolution order is fixed by the rules of the chain (LIFO—Last In, First Out).

According to RiftJudge FAQ #10390, FAQ #9826, and Rule 808.1.d.2, when you use Baited Hook to kill a unit with Deathknell (like your Herald), the Deathknell trigger is added to the chain as a pending item the moment the unit is killed, before the Baited Hook effect finishes resolving.

Because the chain resolves in LIFO order, the Deathknell trigger will always resolve after the effects of Baited Hook have completely finished.

Sequence of Resolution:

1. Activation: You activate Baited Hook by paying its costs and targeting your Herald.
2. Resolution Begins: Baited Hook begins to resolve and kills the Herald.
3. Trigger Creation: Because the Herald has Deathknell, its trigger is created and added to the chain as a pending item before Baited Hook continues (per Rule 808.1.d.2).
4. Hook Continues: Baited Hook finishes its remaining instructions: you look at the top 5 cards and (optionally) play a unit.
5. New Unit Finalizes: If you play a unit from your deck, it enters the board and finalizes. If that unit has a "When you play me" (WYPM) ability, that ability is added to the chain as a pending item.
6. Resolution (LIFO): The chain resolves in reverse order:
   • Any WYPM ability from the new unit resolves first.
   • The Deathknell trigger resolves last.

In summary: You do not choose the order; the Deathknell trigger will resolve after Baited Hook has finished all its instructions.