Bangkok insights
All traveling needs before visit Thailand

Location
Close to Mahakarn Fort at Maha Chak Rd., opposite to Wat Saket on the other side of Rop Krung Canal.
Access by car
Drive to Maha Chak Rd and park in the temple.
Access by bus
Plenty of buses that pass Lan Luang Rd. such as bus no. 2, 8, 37 ect. can get there and with a little walk.
Access by boat
San Saeb canal boat ends its route at Panfa Leelad pier at Mahakarn Fort. It's a good way to get there instead of stuck in traffic jam on the road.
Admission fee
Free Admission but donate in alms box at the Metal Castle entrance as much as you want.

Aerial view of Wat Ratchanadda, Bangkok from Google map