Wat Rakhang temple is located in the south of Wang lang market near the river. My best way is to start from Baan Wanglang Riverside (it's a biggest riverside hotel and bar of that area), the front of this hotel is facing the river, but the other side is located on a small alley in the Wanglang market. This alley is where you can have just about 5 minute-walk along the way to the south (you're going to walk past the Patravadi Theatre) and then reach Wat Rakhang :)

Open everyday, try get there in the morning. Lesser things to see when approaching late afternoon.