There's a lot to include. The key elements to start with would be:

- Purpose & Scope. Define who the policy is for and how it applies to them.

- Travel Time. Clarify whether travel time is paid for or expected as part of a trip.

- Overtime. What triggers overtime and how will it be compensated.

- Accomodation and Per Diems. What sort of policies will you have on hotels and are you going to insist on receipts for all food or offer a per diem allowance?

- Travel class. Will you pay business class?

- Mileage rates. Is there a fixed rate you would pay for fuel per mile? In many countries, there is guidance on what this ought to be.

- Approvals & Evidence. What process should people follow to get approval and what evidence will they need to provide?

- Fairness & Flexibility. Sometimes unexpected expenses pop up when travelling, or people need to come home early so consider how your compassionate leave policy coincides with this.

Make sure you review the policy regularly and apply it consistently to everyone.

Most importantly, ensure that you comply with local labour laws for each country that you operate in. You may need a different version for each country.