Paul Allen flew down to New Mexico where MITS was without ever having tested Basic on a real Altair 8800.
It was on essentially ticker-tape, which was the medium at the time (wikipedia's photo: https://en.wikipedia.org/wiki/Altair_BASIC#/media/File:Altai...)
Paul Allen was still "writing" it on the plane which in this case meant taking careful notes, cross-checking it very carefully and then manually punching holes into the paper. This is 1975, laptops are many years away.
It'd take about 30 minutes to load into the machine. Paul Allen claims they were the longest 30 minutes of his life. The MITS people were just smiling and waiting patiently, eager to see the cool software while Allen, on the other hand, had absolutely no idea whether it would work or not (they wrote it on an emulator after all).
After the 30 minutes it was ready to go and bam, everything worked. Bill and Paul had a company. Microcomputer software was now a thing.
Maybe I should do a computer history podcast ... I just checked and I don't really like most of them ... The video game crash of 1983 for example, I'd do at least 10 30 minute episodes on. The NES came out of it. I mean yeah, at least 10 episodes. I'll pick something super small (for example the HP 9100a) and see how it goes