Unspool a length of spline and install it by pressing it into the frame's groove using a rolling spline-tool (Image 1). Gently pull the spline tight with one hand as you carefully press it into the groove with the roller (Image 2). As the spline goes into the groove, it helps pull the screen taut.
When you reach the corners of the frame where the roller won't reach, use a flat-head screwdriver to press the spline into the corner. Always work in the same direction with the roller -- don't back the wheel up. Be careful not to allow the roller to slip out of the groove as it could damage the screen mesh.