It's very easy. Just send any amount (full number) of steem to @steembasicincome with a Memo of the account you want to sponsor.
Example 1:
Send 10 Steem to @steembasicincome with the following memo: @user1
=> You just sponsored @user1 with 10 SBI (you also get 10 units)
Example 2:
Send 10 Steem to @steembasicincome with the following memo: @user1, @user2
=> Both @user1 and @user2 will get 5 SBI units (you still get 10 units)