You need to detect first, if your mechanism is based on Watt's chain or Stephenson's chain. Then you decide, which of the links you want to declare as base link. This gives you 2 different mechanisms of the Watt chain and 3 different ones of the Stephenson chain (see figures).

Watt-1, Watt-2, Stephenson-1 and Stephenson-3 can be separated into a fourbar mechanism and a two-link subchain. With that strategy your problem is reduced to solve the fourbar mechanism first and subsequently the twobar.

The Stephenson-2 mechanism is said to be not analytically solvable. You are not lost though then, but need to use numerical methods for that.