If you plan your trip to Singapore by flight, normally you will arrive at Changi Airport. In case the flight price ticket is high or fully booked then you can consider on Senai Airport which located at Johor.
There may be not straight forward and take a bit of your time.Firstly, you need to take a bus from airport to JB (Johor Baru) Sentral which connected to Customs, Immigration, and Quarantine Complex (CIQ) . From there will be two different bus company which are Causeway Link and SBS Transit Link.You can hop on these cross-border bus then hop-off at Woodland Singapore Custom. After clear the custom, you can board another bus to Kranji MRT Station, Queens Street at Bugis or Newton Circus.
I will show you how to do it as I experience it before. The most recent was on last week Wednesday.
Upon arrival to Senai Airport,after take your luggage then you can go ahead to the bus counter at arrival hall. Once you out from custom, following the sign board, you will see the KFC and the counter is just right hand side of it. The Senai Airport is not big so you can easily find it. The bus fare just cost you RM8 which operate daily at hourly intervals. The bus is located outside on the left when you exit from the main passenger terminal building.
The journey normally take approximately 45 minutes.The traffic on last Wednesday was good and just took me 35 minutes to reach JB Sentral. Most of the bus stations is crowded with different kind of people, just need to precautions of your belonging.
Once get off from the bus, just follow the signbaord "To Woodland", once clear out the Malaysia custom, follow the crowd to go down and take either Causeway Link and SBS Transit Link. You can purcahse a ticket from the bus conductor. If your destination is Queen Street, then buy and keep for it as you need to use the same ticket for your next ride to Queen Street. Note that, it is just a cross border bus and do take all your belonging once you off the bus. The journey from JB Sentral to Woodland custom only 5 minutes. Try to avoid peak hour to cross the bridge.
Once you hop off from the bus at Woodland CIQ, you can follow the signboard or crowd to passport check counter. You need to fill up a white card before been "interview" by the custom. After clearing everything, then go down from the building and ride the bus to Queen Street. Please note that, if you purchase the ticket from Causeway Link then you cannot ride the bus from SBS Transit unless you pay a new ticket.
Overall, the journey is not straight forward but not complicated. It just an alternative solutions if you want to flight to Singapore or you visit Johor then Singapore.