It is VERY important to unplug your Ender 3 before starting the upgrade process.
Not only is it a safety concern but you will be working with the brains of your printer and any small surges or incorrect connections can fry the motherboard.
With the Ender 3 unplugged, you can slide the bed all the way to the rear of the printer and reach the motherboard housing.
There should be three screws that, once removed, will allow the top plate of the housing to be removed.
Note: The motherboard fan is connected to this plate so you can either unplug the fan now or just carefully lay the plate to the side.
This sounds like a strange step but it saved me a lot of time looking up the proper connections.
Simply take a picture of the stock motherboard connections as a reference.
I found it easier to remove the screws holding the motherboard in place AFTER disconnecting the fans and LCD ribbon cable.
Once this step is completed, the motherboard mounting screws are easily accessible. I had three screws, but there were places for more screws. Your Ender 3 may have more.
I put this as a separate step as it is easier to take this picture after the motherboard is free.
Take a picture of the power connections (black and red wires). I referred to this picture multiple times to make sure I wasn't switching the polarity of any connections.
My SKR Mini E3 came without the heatsinks attached.
The four heatsinks come with double-sided tape already attached. Simply remove the remaining paper and press firmly on the driver chip. Make sure that the heatsinks are not touching any of the other components to avoid a potential short.
It would be very easy to crack the board while installing the heatsinks. Press firmly but not too firmly.
This is where the power connection picture came in handy.
Connect each black and red wire to the same connections that the stock board used.
Note: The 2 wire jack that the woven fabric cables go into on the new board is smaller than it was on the stock board. The left/right orientation is the same it is just a smaller jack.
Take this opportunity to mount the new motherboard with the screws and mounting holes from the original board. Make sure to properly align the micro SD card slot and USB port.
Unless you're using BLTouch on your Ender 3, then you'll need to have your Ender 3, Ender 3 V2, or Ender 3 Pro's print bed on a proper Z-axis to manually level the bed.