Most love hotels do not let you book in advance online (the exception being Bali an).
The way that it works is:
1. Walk into lobby
2. Look at available rooms (displayed on touch screen or physical panel)
3. Select stay duration. This varies between love hotels but there are often 3 choices:
ショートタイム (short time, normally 2 hours)
休憩 (break, normally 3-4 hours)
宿泊 (overnight)
4. Go to counter and pay, receive room plate
5. Go up to room
6. If you are waiting for girl, inform her of room number. When she arrives the front desk will normally call you and tell you that she's coming up.
When you check out, you need to leave together. If one person is leaving before the other, you need to call the front desk and inform them of this. Bring the room plate down to the front and return it. So long as you didn't go overtime/didnt take any of the paid beverages, you won't owe any extra and you can go along your way.