Ring Oscillators (ROs) are established as practical thermal sensors in Field Programmable Gate Array (FPGA) devices. Despite its proven technical usefulness in low-power design and thermal aspects of integrated circuit, a detailed standard procedure is not found in the literature. In this paper we present an optimized scheme including a detailed description of all elements and considerations, so those interested can quickly incorporate these ideas. Also, we show implementation, calibration and resources performing a characterization of a Xilinx Spartan 6 device.