Use SPI_exec... to insert multiples rows in C

I need to insert up to 12 rows (bulk). Basically, new row #1's ID is the
table sequence nextval and parent = null; #2 is nextval + 1 and parent =
row #1's ID, etc. A hierarchy.

I figured the most performant way to do this is to have the sequence
increment by the max bulk insert quantity, 12. This way I can insert all
rows in one statement, which I am sure is faster that 12 insert statements
(I already tried that).

Ideally, I'd like to write this in C using SPI_exec(INSERT...) (or one of
its siblings). I figured it would be most efficient to write the rows to be
inserted into a tuplestore and then insert them somehow. But I don't know
how to do this and cannot find any examples.

Could someone point the way for me.