Commissions can be assigned either during a transaction or after the transaction is completed.
To assign commission:
Step 1: In a Cash sale / booking payment transaction, click on the item to reveal the options popup.

Step 2: The edit popup will appear
Step 3: Select the team member you wish to assign the item's commission to and tap Save

Step 4: Once selected, the name of the employee will appear below the product/service, with 'assisted by' next to their name.

Note that the commission rate will depend on whether you have setup default commission or Team Member's commission rate.