New to linkages? For some easier builds like Punchers, Jumpers and Biters check out Linkage Warm Ups.
If you want to create a custom linkage for your walker like this 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
Trouble Shooting Tips: A finished walker's legs should be easy to rotate by hand with the motors disengaged. If you feel resistance, then look carefully at the legs to see if any are twisted due to leg parts connecting to other parts in the wrong plane . Also, make sure that all bushings have been added to the axles so that the legs and cranks don't collide.
Below are two Linkage Warm Ups builds: