Go to Packages and choose "Create Package". Set a title, a session count and a price; you can also add a description and a usable period in days (leave it empty for no expiry) and choose who can buy it.
Open the client's card and use "Add Sessions" to pick a package and grant the sessions straight away. This is recorded as a cash sale, and is separate from a member buying a package themselves on your booking page.
Each package has a "Who can buy this package?" setting: "Everyone" shows it to all members on your booking page, while "Specific clients" limits it to the clients you choose. You can still assign any package to any client from their card, regardless of this setting.