Also see here :   https://treasurydirect.gov/indiv/research/faq/faq_irstaxfeature.htm

Using Your Income Tax Refund to Buy Paper Savings Bonds

The Series I savings bond is the only savings bond available in paper form, and it can be purchased only through this program.

(Series I bonds also are available in electronic format, in TreasuryDirect.  This page addresses paper I bonds only.)

What do I need to do?

When you file your tax return, include IRS Form 8888. Complete Part 2 to tell the IRS you want to use part (or all) of your refund to purchase paper I bonds. Purchase amounts must be in $50 multiples and you can choose to have any remaining funds delivered to you either by direct deposit or by check.  You do not need to open a TreasuryDirect account; just follow the instructions on the form. Once your tax return has been processed by the IRS, your paper savings bonds will be mailed to you.

How much can I buy?

In any single calendar year, you can buy up to a total of $5,000 of paper I bonds using your refund. You buy I bonds at face value, meaning if you pay $50 (using your refund), you receive a $50 savings bond.