See the Nemo wiki. In essence, if you already have a working U-Boot, just write the image to the SD card using dd or whatever tool your OS uses for doing this, put the SD card in your N900, boot with keyboard open, go to the console, and type in "run sdboot". Note that if you're trying to use it as a rescue system, it doesn't contain mtdutils (and for some reason, it wouldn't connect to my WPA-PSK network properly...)

Edit: Oh, and the RescueOS issue was a major oops on my part - it's not actually a bootable image, but an initrd file. Anyways, I'll reformat the card to fat32 and try again, but is there any way to specify the use of an initrd file to U-Boot for an SD boot?