By default, the taxes you enable in your settings will apply to all services & products that you charge within your business. To modify the taxes to your services or products follow these steps:
Taxes on Services or Products
Click on the appointment and select the Mark As Done or select the POS option
Click the "Done" option at the top left portion of the screen

If you have your Taxes added already they will automatically be applied to the service
If you wish to not charge certain taxes for certain services you can de-select those taxes to be applied.
If you like to add a TIP, you can add this later in the POS workflow
Select the Edit icon for the line item service or product you'd like to edit the taxes for:

You can then enable or disable the taxes you'd like to charge for this product

You can review to see the total amount and the charge of the taxes and tips before you mark the appointment as completed.
When finished editing the applicable taxes > Select the Pay option to proceed with capturing the payment for this service or product