This is what I just did last weekend to clean and fill a 1/6 bbl sanke. It was my first time doing it and I found it fairly straight forward but then again I researched the crap out of it first. I highly suggest doing a few practice runs filling and purging the keg to get the hang of things. I used water to do this.
1. Remove the spear. Lots of info out there on how to do it but this is a good video to get your started
https://www.youtube.com/watch?v=1ZPfZ_AL380
2. I used a DIY carboy/keg cleaner to do the actually cleaning. There are some good threads here on how to make one.
3. Rinse keg and let air dry. Once dry cap the keg until you are ready to fill.
4. Disassemble sanke spear, soak in PBW. How long depends on how dirty it is. After it soaks, rinse and let dry but do not reassemble. Keep it covered. I soaked mine in a wall paper tray and then covered with saran wrap.
When it comes time to fill (which I will explain later) this is what I do:
1. Mix up some no rinse sanitizer. I pour about a 3 quarts into my keg, cap the keg some how to to keep the sanitizer in. Roll keg around on its side to make sure all the walls get sanitized. Hold upside down and splash around the sanitizer so it covers the top. Hold it right side up and splash around to sanitize the bottom. Pour out sanitizer. Cover the opening until you are ready to put the spear back in.
2. Let the disassembled sanke spear soak in sanitizer, reassemble spear and put it back into the keg. Again reference the video on how to do it. I would practice this a few times to get the hang of it before you actually fill.
For filling, I use a modified sanke couple with both check values removed.
1. Connect the coupler to the keg and push the handle down.
2. I used an auto siphon to transfer from my carboy to the keg via the beer line of the coupler.
3. Once filled, place a clean and sanitized penny in between the washer and barb of the beer in line of the coupler.
4. Hook up CO2 tank to the gas in of the coupler and purge the air from the keg. Lots of info on here about purging a keg.
One trick for taking the spear apart is to get a 1" PVC tee fitting, cut notches in one end that align to the flared retaining ring. You can then use this to push down the retaining ring and twist at the same time to remove/install the retaining ring.
If something doesnt make sense just ask and I will try to clarify the best I can. I am picking up another 1/6 bbl this weekend so I can also take pics then if that would help.