Instructions for mounting a 120mm fan on the Ninja:
1. Loosely attach one of the wire clips to the fan, and slide the long "bar" into the notches on the upper side of the heatsink (near the power supply).
2. Pull the fan around the heasink so that the fan is flush against the heatsink, and the single clip is under tension. Hold it in place with a single hand. The remaining steps will be easier if the unclipped edge of the fan is aligned with the edge of the heatsink; this will make the fan slightly off-center, but it can be slid into its proper position once the second clip is in place.
3. Attach the lower end (closer to the motherboard) of the second clip to the fan.
4. Slide the middle of the "bar" of the clip into the lowest notch on the heatsink.
5. Pull the bar up through the row of notches until the clip is aligned properly and keeps the fan in position. This will bend the lowest fin of the heatsink slightly, which is normal. The fan should now be held in place on three of its four corners, and you should be holding the remaining "prong" to prevent the clip from sliding back down the side of the heatsink.
6. The last step is to secure the remaining corner (at the top of the heatsink in the middle of the motherboard where there is lots of room to work) in place. This is the only step where you will need to use much force. Make sure the "bar" of the clip is in the notch on the uppermost fin of the heatsink, and press the L-shaped corner of the clip against the fan until is slips into the mounting hole in the fan. The clip will probably be bowed out and will not stay in the notches of the middle fins. This is normal and is perfectly secure so long as the clip is in the top and bottom notches.
Hope this helps. Let me know if any of this is unclear.
Yep, that's exactly how I've been doing it, but I can't figure out how to avoid needing a third hand around steps #5 and #6. All that pushing means the mobo will be sliding across the table and
you'll be applying a lot of torsional stress to the heatsink retention mechanism and mobo.
I've done it once or twice by myself, but have given up on that and just have my wife hold the board in place (and support the top of the heatsink a bit, too) while I hold the fan and fasten the clips. It's about 4,000,000 times easier that way, and a whole bunch safer but unfortunately, it's not at all "macho".