How to connect power and fan to RAMPS 1.6?
You only need to connect one pair (one red, one black) of sufficient wire gauge to the single power connector's plus (red) and minus (black). There is no need to bundle multiple pairs from the PSU. Some pinout graphs are not clear in this respect! I've always connected the heated bed through an external MOSFET to keep large currents away from the board, but according to the design, it is capable of handling the current.

E.g. this is the correct pinout:



And this an incorrect pinout graph (look at the lower left where the power needs to be connected to the board, this hints to bundling the 11&nbsp;A and 5&nbsp;A wiring):



The part cooling fan should preferably not be connected to a (non-schedulable) 12&nbsp;V header. Instead you should use the D9 output connector to schedule the airflow of the fan for part cooling.

Note there is no spare 12&nbsp;V header available on the board. You could use the spare PSU pair to power the coldend (radiator) fan.