Basically, do it the way Spivak does. Funny thing is in freshman year we were taught to do just that, but then abandoned the formality pretty much immediately after.
My reading of that is that Lamport found faults in Spivak's approach. Did I missread that?
I will say that this really looks like a way to write proofs for computers to read, moreso than a way to write them for other people. Probably has great value, but not too surprising that it isn't widespread.
http://research.microsoft.com/en-us/um/people/lamport/pubs/p...
Thankfully, his view underpins elite mathematics programs, e.g.
> Math 55 assumes students can write a tolerable prose proof.