If you want to make a beer and serve it out of a sanke keg... you gots ta do it in a few steps.
first... you gotta clean the sanke. The only way I know of to clean a sanke (assuming you don't own a professional brewery keg-cleaner) is to take the dip tube out and clean it by hand. I ferment in un-cut sankes and have taken all of the dip tubes out of mine. It takes about 90 seconds and you can easily google how to do it. (if not, let me know and I'll tell you). To clean it, what I have done is a several step process.
1) rinse it out as well as possible with the hottest water you can get out of your tap
2) fill it with same hot water and PBW... all the way to the top... let it sit overnight.
3) Rinse it, add a little more hot water, more PBW and systematically scrub in the inside with a carboy brush
4) rinse really well
5) put two gallons or so of water in it, place a piece of tin foil over the top, poke a tiny hole in the tin foil and then place the whole thing on your turkey fryer (assuming you have one) and let the thing heat up adn then shoot steam for a good 20 minutes
6) rinse... Star san & water... shake shake shake
7) flip over and let dry
... I think this is all just a little overkill but that's how I do it just to make triple sure you're not picking up any crap from the old beer that was in it.
Second... you need to fill the sanke with your beer. That's easy... just siphon the beer in.
Third... you need to put the dip tube assembly back in. AFTER THAT IS ALL CLEANED AND SANITIZED. If you took it out, you'lll know how to put it back in.
Fourth... Pressurize the keg. You'll need a CO2 tank and the sanke coupler that is pictured above in another post. Simply tap the keg with the coupler and force carb it like you would a corny keg. NOTE: Obviously make sure you have a picnic tap or something on the "beer-out" line of the coupler or you'll just spray beer all over the place.
Five.... tap keg and drink beer. for this you can either use the CO2 tank and coupler set up you used to pressurize it OR you can simply use a hand pump. If you're going to use the hand pump, you just need to finish the whole keg in... pretty much... one sitting because as soon as the air gets to it, it'll start funkifying. If you use CO2, it'll be good for a long long time.