Reservation consists in pre-purchasing capacity by committing to its use. This has the double advantage of reducing the cost of the settlement and guaranteeing the availability of the reserved resource at the supplier. But in return it reduces the flexibility of on-demand.
As for rightsizing, it is possible to do the calculations yourself via spreadsheets to define the bases of instances to be reserved or to use a tool.
Please note that this is not necessarily intuitive: Sometimes the right choice is not to keep the smallest classes of instances even if it means reserving less of them. Sometimes it is better to put bigger instances and reserve them. It is often cheaper to reserve a larger instance than to take smaller on-demand. The CMP (Cloud Management Platform) on the market are able to find the perfect combination thanks to their sophisticated calculations and can also offer different options (1 year, 3 years, full upfront, patial upfront or no upfront but also convertible or non-convertible instances).
Once the list of recommendations has been made, it is necessary to analyse them and validate the changes. Then make the validated purchases via the supplier's console: