How do you reimburse an employee with petty cash?

you should make it in legal way to aware any problem and circumstances or any question from your head simply make a reciept dated the day you re inburse with the amount how much...and make sure with the authorise signature,and signature of that person who recieved.

first the custodian have to fill out the form number/Name of the employee,also have to jot down a brief description of the purpose of the cash given,lastly the person responsible to GIVE OUT the money and who has the authority of the petty cash has to sign in followed by the person who has been the beneficiary of the money 

exactly the same way described by Abhishek Desai  

Certified Petty Cash Custodians Employees present the custodian with expense receipts authorized up to $100.
Reimbursement of petty cash requires authorized, validated receipts.