Here’s how to convert an old Suntour Perfect 5 speed cluster into a single speed 17 tooth freewheel. These sprockets are usually made of harder steel than modern ones so they tend to last a long time. They are more brittle though and I often see teeth broken off (presumably from bad gear changes).
So the process for a Suntour Perfect 5 speed cluster is:
(1) Secure chain whip in vice and wrap around 3rd sprocket
(2) Wrap second chain whip around smallest sprocket and rotate counter clockwise (regular right-hand thread)
(3) With great force the smallest sprocket will loosen and unthread from the cluster body
(4) The second smallest sprocket can then be removed in the same way
(5) The larger 3 sprockets will slide off like a cassette. Don’t turn these, just push them directly out.
(6) Now you’re left with the main cluster body and the individual sprockets. First slide the small spacer on and then either of the 2 smallest sprockets, usually 14 tooth and 17 tooth sizes. They will tighten against the larger end of the cluster body.
! Note that not all clusters have the 2 smallest sprockets threaded on like this. For this reason, this particular type is most suitable for the conversion.
Cycle Saloon 
This is a great tutorial. I’d like to mention that it can be extremely difficult to break off the locking sprocket. I’ve broken numerous chain-whips trying, with limited success. That being said, i think its a great use of those freewheels that would otherwise end up in the recycle bin. They can be of better quality than a cheep replacement ss freewheel.