I'll describe what I did. There are a couple of ways to do this, but this is what works for me:
First, you are going to allocate five consecutive memory channels on your radio to the satellite. You need to do this to accommodate the doppler shift effect on the 440Mhz band.
The primary downlink frequency on AO-51 is 435.300. The uplink is 145.920, with a PL of 67Hz. Because of the doppler effect, you need to shift your starting listening frequency
higher than the main downlink frequency and then shift it lower as it passes over.
Look up in the manual how to program Odd Split memories. In my VX-8 manual, it is page 48. Like I said, you should use 5 memories, and label for easy recall. I labeled mine according to the satellite name, with an a,b,c,d,e,f following to tell me where to start. So, I have 5 consecutive memories labeled A0-51a, A0-51b, A0-51c, A0-51d, A0-51e. On the VX-8, page 50 tells you how to label memories. The transmit frequency is the same for all 5 memories. I start the first receive or downlink frequency 10Khz above the downlink frequency on the first memory that I labeled A0-51a. That works out to 435.310 Mhz. Then on the next memory channel (A0-51b) I set the RX frequency to 435.305Mhz. Memory A0-51c RX is 435.300, A0-51d RX is 435.295, A0-51e RX is 435.290Mhz. Remember, the TX frequency is the same on all 5 memories (145.920, with a PL of 67Hz).
One last thing I do to help organize the memories and make it easy to find is set up memory banks for each satellite. Once you have it set up, when you select the bank for the satellite you want, it makes it so you can't accidentally change to other frequencies. Not all radios can do this, but it's a nice feature of the VX-8. Check page 53 in the manual for details.
Programming the memories and banks is pretty tedious, but it is WAAAAAAY easier and quicker if you purchase the programming software by RT Systems. I highly recommend the software if you are planning on putting in all the satellites and other repeaters in your area.
Now, when the satellite is low on the horizon, start listening on the first memory, pointing the antenna at the satellite. On my radio, the first one is labeled A0-51a. As the bird makes it's pass, you will hear it start to fade. If moving the antenna doesn't bring the bird back into your RX, change to the next memory. AS you progress through the memories, you should find that the middle (A0-51c) memory will be at the apex of the pass. Sometimes you have to quickly change back and forth between memory channels to find the one that works the best at a particular time.
Remember, keep your squelch wide open while trying to work the satellites. Using headphones is also a very good idea.
Like I said, take a look at the pages I pointed out in the manual and if you need help with something specific after trying it out, let us know! Be sure to fill us in on your progress as you try this out.