You can reach Nicaragua on different budgets and times. We went on one somewhere in the middle option. I'll explain the details below.
Booked ticket at the bus terminal from the shopping Mall in Monteverde. The bus is leaving to La Irma at 4:20 AM. if you are on the route of the bus they will pick you up from the hotel but you need to let them know in advance . Price is 1050 colones per person (approx. 2$).
After 2h, the Bus drops you off on the PanAmericana. Cross the 'street' as you need to get the bus in the other direction.
Here you can take any bus going towards Rivas (Just make sure it does not pass Nicoya). You can either get another bus to Penas Blancas and from there one to Rivas or get into a Tica or Transnica bus which will bring you directly to Rivas. If you want to save some time and have some money to spend might be better to get one of these buses. We took the Transnica (as it came first) it was 20$ per person. This option also saves you some time at the boarder as they arrange everything for you.
At the Boarder (Costa Rica - Nicaragua)
Exit Tax from CR is 7$ (there will be a lady trying to convince you to go with her to sort everything faster. This is just a lie, it will cost you 8$ and then again it won't really save you any time. you can pay your tax inside at a machine. You need a credit card for it.
You then get back in the bus and they will collect all your passports plus a 13$ tax for entering Nicaragua.
Drive a bit and once again, get off the bus with all your stuff and you need to pass through a sort of control like at the airport they get your luggage trough a security machine. Once done the bus will be waiting outside.
Keep an eye on your stuff until they finish cleaning & disinfecting the bus. There will be loads of locals trying to sell you everything and to try to convince you to exchange, money, ignore them. We did buy a coconut as it was super cheap :).
And guess what, you go in the bus and then again off the bus waiting for your passport. Someone from the customs office will come at the bus and bring you back the passport.
This takes about 1h altogether. Avoid getting there on weekends as there might be longer queues.
From there the bus goes further to Rivas where they drop you off or continues to Granada if that's where you want to go. (this particular one was going till Honduras)