Are you designing something, or modeling an existing system?
If designing, break your task down into steps. Sketch a bunch of straight lines and apply some constraints. Move the movable stuff and see what moves. Fiddle dimensions around until you get he behavior you want. Now, you can model real parts!
If you are modeling an existing linkage, model the pieces and assemble them. IF it does not work, you have modeled something wrong. Maybe you don't understand the linkage.