Speak to the local RIR[1]. They have varying requirements, but broadly you generally need to justify your use case, multi-home (for your own AS) and pay a yearly membership fee. After that, you need to speak to your ISP about either advertising it or peering with you - or going dark fibre if you're a real masochist.

Good luck, update us if you do it!

1. https://en.wikipedia.org/wiki/Regional_Internet_registry