If Hamlet kills Claudius while he is praying, Hamlet fears that Claudius will enter Heaven and find eternal forgiveness. Hamlet does not desire to have Claudius forgiven of such a cruel crime as killing King Hamlet.
Perhaps you could write about Hamlet's feelings while overhearing his Uncle Claudius praying. Perhaps you could begin with Hamlet's emotions while he overhears Claudius praying. The following would be an example:
Oh how I hate his prayers!
He prays so reverently when he should die violently.
Oh how I wish to divide his prayerful heart with my sword!
In haste, I must not kill him now.
I must patiently wait for him to sin against God and Heaven.
Then shall his guilty heart receive my sword.
(Perhaps, the above has helped you know how to begin).
(Continue to write how that Hamlet must be patient in his desire to avenge his father's death).
(Continue to write how Claudius' death must be met with sin in his heart).
(Write about the decision Hamlet has to make in waiting until Claudius is in a drunken, sinful, angry state).
I hope this helps you get started. Good luck!