Writing ASM library to work with QB...

I get assembled and libbed procedure to work with QB...
But not with exchanging parameters or variables or datas...
I tried:
Public testme
Proc testme
Push BP
Mov Bp,SP
Mov DL,[BP+06] ;Get the variable passed from QB...
Mov Ah,06h ;I think this function is used to display a byte...
Int 21h ;OKay, I passed one variable to it and interrupt it...
Mov [BP+06],000Ah ;Well, I wanna pass back some parameters
;Is it passed to the function as a return value or a variable???
POP BP
RETF ;Should I put any value behind Retf,if yes, what is it?
testme ENDP
;Heck it won't work
Anyone willing to help me? Pls Write a demonstration version...