New to linkages? For some easier builds like Punchers, Jumpers and Hoppers check out Linkage Warm Ups.
If you want to create a custom linkage for your walker like the below simulation of Strandbeest's linkage optimized for LEGO, see Customize where you can find online optimizers for each mechanism.
Strider's linkage plans can be found here.
TrotBot's linkage plans can be found here.
Scott Anderson created his own 3D-printed, phone-controlled version of TrotBot - you can find his plans here.
UPDATE: for improved functionality for Mindstorms Strider Ver 3, see these instructions for mounting the EV3 brick higher