How do I appoint a sole executor that if unable/refuses to act, is substituted by joint executors instead?

Under the Appoint People tab, under Appoint Executor, choose your sole executor. Leave the Substitute Executor section blank.

Under the Select Clauses tab, toggle on Other Instructions and key in the following:

Appointment of Substitute Executors

Should my {Relationship}, {Name}, {ID No.}, die before me, refuse to act or is unable to act, or his/her appointment as my Executor does not take effect for any other reason, I appoint my {Relationship}, {Name}, {ID No.}, and my {Relationship}, {Name}, {ID No.}, to be my joint executors and trustees (hereinafter called "my Substitute Joint Executors") of my Will.

If you want to have additional substitutes as a standby to replace your substitute Joint Executors, you can add in the following clause:

Should any of my Substitute Joint Executors also die before me, refuses to act or is unable to act, or his/her appointment does not take effect for any other reason, I appoint the following personnel in order of priority to replace him/ her:

(a) {Relationship}, {Name}, {ID No.}
(b) {Relationship}, {Name}, {ID No.} …