Need help in wiring two Peltier Plates together
The first thing that you will need to do is find out how much power your device needs. The peltiers need to generate more power than the load consumes. So if you need 1W for a fan, the petlters better be able to generate 1W. You need more voltage than the load needs. If it's a 5V fan then the peltiers need to be able to go up to 5V.

The second thing is you need a temperature gradient (or temperature drop across the peltiers) This means one side should be hot and one side cold. Use this datasheet to find out how much the temperature drop. The datasheet also shows how much power you can generate. I'm not going to go into further explication, because I'm willing to bet this is an academic exercise and some work needs to be done by you. 


If you have a with the hot side at 50C and the cool side at 30C, you'll have 0.5V with a matched load (~1Ohm). If you want 3V, you'd need the hot side to be at 200C and the cold side to be at 30C. These peltiers are not the best to use if you want voltage.

If you connect the peltiers in series, and they will generate twice as much voltage than one peltier alone. If you connect them in parallel they will generate twice as much current. 

Use thermal paste with a low thermal conductivity to connect the peltiers to the heatsink.