Select the item(s) for purchase on the Sell screen. Add a customer to the sale (you may need to create a customer first under the Customers tab).
Next, click pay, enter the amount of the deposit/payment, and click the payment type to apply this amount.
Then, to save as a layby, click "Layby", add a note (if required), then click "Save".
The amount tendered (either as a deposit or part payment) will appear under it's corresponding payment type, when you close the register at end of day.
Further or final payments can then be applied to the transaction at another date by re-opening the sale from the Retrieve Sale tab.