Go to https://polkadot.js.org/extension, download and install the extension accordingly with your browser.
Open the extension, and click Understood, let me continue.
Note: Without doing this, app.neukind.jp will not able to launch the extension
Click the "+" plus icon in the middle of the extension
Write down your 12-word mnemonic seed and keep it in a safe place. This is extremely important! You will need the seed to recover the wallet later.
Input name, password for your account. Please save your password in a safe place, too. It is used to import/export your account.
To backup your account, click three-dot on the right side of your account and choose Export Account. Then, input your password and click I want to export this account. It will download a JSON file. Please keep this JSON file secure. You will need it along with the password to import your accounts to the extension later.