The first step to processing an order is to find it – there are several options for this in Kartris:
The 'Orders' menu - this can bring up a list of orders filtered by various criteria such as recent, awaiting invoice, awaiting payment and so on.
The main back end search box can find orders by ID.
The search box on the orders list page can find orders by ID or date.
Orders requiring attention (generally new orders) will show in the 'To do' list on the right hand side.
You can click to view a customer's orders from either the customer listing, or the customer detail page; so if you need to find an order for a particular customer, you can find the customer by email or name (or part of) and then from there find their order(s).
In Kartris 1.4x onwards, the order IDs on the customer's order history tab are also clickable.