Singapore does not allow a direct bus service as the government view Senai Airport as competition to their own airport.
Air Asia have a hub at Johor Bahru so if you want to go to Singapore it may work out considerably cheaper to fly to JB. A saving of a couple of hundred dollars on a flight makes it worthwhile for the time it takes to transfer from JB to Singapore.
[Air Asia at Johor Bahru Airport]
How to get from Johor Bahru airport to Singapore
At Johor Bahru airport take the bus to Larkin Bus Station, which is the bus terminal in Johor Bahru. You can buy tickets for the bus in the arrivals hall. Tickets cost RM8 one-way and journey time is approximately 45 minutes.
At Larkin Bus Station get the Causeway Link Express Bus (look for the yellow bus). There are two services that run to Singapore:
1. CW1 Larkin Bus Terminal <--> Kranji MRT Station
2. CW2 Larkin Bus Terminal <--> Queen Street
[Causeway Link Bus]
The Queen Street service is the most useful bus for visitors but you can get on either service to start with.
The bus then takes you to Johor Bahru Customs where you will alight to clear Malaysia customs.
Once you have cleared customs return to the bus park and get the next Causeway Link bus using the same ticket. This will take you across the causeway to the Woodlands Custom terminal in Singapore.
Once you have cleared customs in Singapore exit to the bus stop and get on the Causeway Link bus. This time you will need to get on the correct bus number (CW1 for Kranji MRT Station or CW2 for Queen Street).
By now you have two more stamps in your passport and the bus will take you straight to Queen St bus terminal.
All up you will need to allow for around three hours from arriving at Johor Bahru Airport to arriving at Queen Street Terminal in Singapore.
[Causeway Link Express Bus]