We demonstrate a facile one-step fabrication of flexible ridge waveguides by using the nanochannel-guided lithography (NCL) that enables continuous extrusion of a polymer that forms waveguiding core ridge on an undercladding polymer substrate. NCL utilizes a well-cleaved mold edge with microtrench patterns to slide continuously over a UV-curable liquid resin-coated substrate under conformal contact, where the resin and substrate can be chosen for suitable waveguide core and undercladding materials. The local heating of a trench mold can control the viscosity of liquid resin for optimal filling into the microchannels followed by smooth extrusion, which is subsequently UV-cured. Such a smoothly extruded resin core exhibits a very smooth surface potentially promising for the low-loss waveguiding operation which was experimentally confirmed by optical insertion loss characterization. The proposed technique may provide a practical route to the continuous and seamless fabrication of scalable waveguides and photonic elements.