Go to Products Page. Click on any product.

Variation options will be displayed in the single product page.

Select the options and Go to Checkout.

Delivery Slots will be displayed on the checkout page:

You can add products from the shop page and go to checkout.
Delivery slots will be displayed on the checkout page.
Select delivery date from the date slot.
Select delivery time from the time slot.
From the bookings page you can reserve the delivery slot in advance.