How do I fill out a counseling form for a soldier who returned a day late from leave?

Use a keyboard. Or use a pen if you are more old school.

The counseling form purpose will be labelled as Event Oriented. In the Body, just simply state what the counseling is about, as in what happened. In the Plan of Action, outline what the (if any) corrective training will be.

What might you need help with?

Difficult to answer without additional information. For example:
Were there extenuating or mitigating circumstances? (Airline or vehicle issues)
Did the Soldier notify anyone in the COC/NCO SC, or SDNCO to request an extension of leave?
Are you planning on referring the Soldier for UCMJ? (Soldier has history of poor performance and this is another in a series of infractions)
Do you simply want to inform the Soldier of the leave policy, and possibly implement some corrective training? (Educate the Soldier, and have documentation should it become a pattern)