Whoa... slow down - nothing needs to be cut!
Hopefully you bought a low-profile NIC - these may come fitted with a full height bracket and a low profile bracket in the box, attach the low-profile bracket before proceeding. If you have purchased a full height NIC, return it for a low profile card.
To install, undo the two blue scews at the front of the motherboard - undo them fully, until they pop up on their springs. Remove all the motherboard connectors (drive cable, fan, 2x USB, power). Slide out the motherboard tray.
On the back of the server, flip down the flap that is located below the PSU fan vent - this covers the PCIe retaining screws. Remove the screw for the left PCIe slot (as you look at the back of the server) and remove the silver slot cover.
Install the NIC into the PCIe x1 slot, and re-install the tray. Replace PCIe retaining screw. Job done. If the tray doesn't go all the way in, it's likely one of the blue spring loaded screws at the front is "catching" on something so pull them up when sliding the tray into place.