Step-1:
In reverse order, The hydroxide ion deprotonates the aldehyde.
Step-2:
Here Enolate ion 1 adds to the unreacted aldehyde.
Step-3:
Alkoxide ion 2 is protonated by water.
Step-4:
A small amount of aldol is converted into enolate ion (4) by hydroxide ion.
Step-5:
Here Enolate Ion(4) loses a hydroxide ion.
Step 1 to step 3 illustrates the aldol reaction.