We report on micro fabrication methods used to integrate electronics into smart textiles. We have introduced a shadow mask fabrication method to pattern 2D thin film structures onto the 3D surface of yarn. We have demonstrated the fabrication of gold resistance temperature sensors (RTDs) onto 500 μm thick nylon yarn. The sensors are meander shaped and have lateral dimensions of 0.3 mm × 1 mm and the width and spacing of the meander is 20 μm. Two different fabrication methods are developed and evaluated: 1) a planar mask made of Kapton foil is patterned by dry etching and 2) a tube mask made of Kapton foil is patterned with a laser. Both methods have proven to be suitable for the fabrication of RTDs onto textile yarns. The fabricated sensors are measured in a climate chamber from 15 to 50°C. The temperature coefficient of the RTDs is 0.0026 ω/°C with a sensitivity of 0.74 ω/°C.