With Tenso, you are given a mailing box (So basically a specific address which you use as a shipping address) so the process will amount to:
Signing up with Seagull (or the store in question) if you haven't an account with them. This may require you to get through a process in Japanese, and the only advice I can give is 'Copy/paste a lot of stuff in Google translate so you can determine what fields are what'
From there, your shipping address for your order is set as the Tenso address as given, and if possible, the billing address is your real one, for the following reason.
When asked to provide a method of payment, you're required to match the payment address with the address on file for the card/paypal etc.
Then the process should work.
If you don't want to go through all the manual process, I understand there's assisted shopping, but there are additional fees due to the higher level of service.