When you're ready to place an order, you can open a detailed view for each of the products by clicking the cart icon next to the product. This opens a window which allows to learn more detailed information about the product.
In the top, you will find the alternate suppliers which you can set by clicking Edit suppliers button, as well as settings that you can adjust on the fly to affect suggested quantities:
Lead time
Order stock for (Days of stock)
Then in the middle section you will find a table with detailed breakdown for:
Sales history for different sales periods
If and how long the product has been out of stock
Estimated order quantities based on your settings
Once you've made a decision and have placed order for said product or added it in a purchase order, enter the quantity in the Quantity ordered field and press Mark as ordered button.
This will change the status to Ordered and move it over to Ordered tab in the table to get it out of your way so you can focus on the other products that require your attention.
You can always remove this status by opening the detailed view again and clicking Remove order button.