Try this:
For it_tab create a table with the same structure as how you selected the data below.
SELECT a~rbukrs
a~racct
a~tsl
a~poper
a~gjahr
a~belnr
b~kunnr
b~belnr
b~bldat
b~xblnr
FROM faglflexa as a
INNER JOIN bsid as b
ON b~bukrs EQ a~rbukrs AND
b~gjahr EQ a~gjahr AND
b~belnr EQ a~belnr.
INTO TABLE it_tab
WHERE a~rbukrs IN s_bukrs AND
a~gjahr IN s_gjahr AND
a~poper IN s_monat AND
a~racct IN s_saknr AND
b~kunnr IN s_kunnr.