I dont recommend Pocket WiFi in Japan unless you really have to.
Go to website and fill in some details. Make sure you check how much data is available for 3 days / 7 days / per month. Once you used over this limit. They will speed limit to 200Kbps ~ 1Mbps (unusable).
Rent 7 days, 2 weeks, 3 weeks probably same price as renting for a month.
Once delivered. The Pocket WiFi usually has SSID (name to connect to WiFi) and password (they often call KEY here).
Either Konbini or Post Office (ゆうパケット).
NETAGE gave you a return ゆうパケット with address sticker so you don’t have to write. Consider this is a ¥500 discount worth.
If you forgot to return, usually the contract automatically extended.