Create a full-detail schematic of the system in version-controlled UML.
At some point, "deploy" the UML by printing it into a 4 cm-thick binder of paper, then distribute these binders to the head architects.
Iterate on the UML until the architects are happy. (The architects spent many years trying to auto-generate code from the UML diagrams and have the results "fleshed out" by lowest-bidder consultants, though this never really worked. Their stated goal was to no longer have to write any code in house, but rather nothing but UML.)
Begin implementing the system in house with auto-generated code from the binder-of-UML as a baseline, after the lowest-bidder consultants had failed.
Quickly get into big fights between the coders-on-the-ground and management when it was found that the UML diagrams contained major architectural flaws and the UML-phase would not, actually, account for 80% of the project's duration and budget. Needless to say, more than half of the projects failed entirely.
This experience nearly made me leave the industry, before I discovered that there was plenty of software being written in a saner way. This was more than a decade ago, but to this day, just seeing UML diagrams turns my stomach.