You'll need to create a Japanese PSN account. Instructions to do that are here: https://japancodesupply.com/pages/how-to-create-a-japanese-psn-account

After that, you will most likely need to purchase a Japanese PSN prepaid card (unless you have a compatible payment method), and find the game normally as you would any digital download.

Note that there are two versions on the PSN store...the SD version and the HD version. You'll want to download the HD version if you plan on speedrunning the fastest version.