To convert the used beer kegs into useful brewing vessels, they need the tops cut out of them. Google search and YouTube videos show all kinds of complicated ways to do this, including things like using a jig to hold the grinder in the right position or even needing a plasma cutter. Since I had not done anything like this before, I did try to follow some of the advice, but found most of it pretty useless and the actual procedure much simpler and easier than I imagined.
In the pictures below, the top collar of the keg has been removed, I find that this makes the kegs easier to work with and it also makes them lighter, the handles still retrain their full strength even with the rest of the collar cut away. Removing the collar before cutting the top off the keg gives more working space, which is useful in keeping the grinder upright, especially when the cutting disk gets worn down. However the collar does make a useful guide and can help cut the circle more cleanly.
I was planning to use this keg as both the HLT and still-pot, but SWMBO was not keen on the still-setup and installing the various fittings for the HLT would be significantly harder with only the spear-hole to work though, so I decided to cut the top out of this keg too - and took some pictures as I did.
The only major tool needed to cut the keg-tops is an angle grinder, a thin steel cutting disk and a thicker steel grinding disk are both need, and will likely be destroyed in the process, however the $1 disks (shown below) worked well.
Continue to work the angle grinder around the groove, trying to keep the cutting depth even all the way round, this should help preserve the cutting disk and if you do not concentrate on one section for too long it allows the stainless steel to cool before you come back to cut on that location again.
Run the grinding disk over, around and under the hole to smooth it of and remove any jagged bits of metal that were left behind. The best method I found was to rest the guard of the angle grinder on the edge and then guide it backward grinding and smoothing the edge gradually around the circle - this allowed a good level of control and also ensured that the (generally) circular hole remained (pretty much) circular.
After a bit of work - but nothing too difficult or strenuous - the top of the keg has been cut out, smoothed off and fits the lid exactly: