To install the impeller lube the entire exterior with the supplied lube (usually glycerin). Avoid the use of petroleum based lubes as they may shorten the life of the impellers compound. After lubing compress it with your fingers aligning the vanes in the direction they were oriented when the old one came out.
I often hear of folks talking of using zip ties to pre-compress the impeller for insertion but I honestly don't understand why one would need to do so. I've replaced impellers on very large Detroit diesels, Cummins, Caterpillars and even one lung teeny tiny Yanmar's and all of those impellers went back in fine without a zip tie pre-compressing it. You really do not need to over think the re-insertion just do it.
If your impeller had vanes pointing in different directions when you took it out, as the photo of the old one shows, DON'T WORRY. This is rather normal and is due to the little bit of "back spin" a diesel can get as it shuts down. Sometimes during the last revolution the engine may actually spin back a notch or two causing the impeller blades to be out of spin direction unison. Even of you put it in completely backwards of the pumps rotation it would correct itself with no long lasting trouble. I don't advise this but the reality is that it won't actually hurt much.