I dug the following code out of my archive (it's dated 24/06/1995); 
this is the keyboard handler for a program which I wrote which blanks the screen if both shift keys are pressed.
kb_int      proc    far
            pushf                           ;Save FLAGS
            push    ax                      ;Save AX
            in      al,60h                  ;Read the scan code
            mov     cs:[scancode],al        ;Save it
            pop     ax                      ;Restore AX
            popf                            ;Restore FLAGS
            pushf                           ;Push FLAGS
            call    cs:[int09h]             ;Call previous handler
            sti                             ;Enable interrupts

            test    cs:[scancode],80h       ;Exit if high bit of scan
            jnz     kb_exit                 ;  code is set

            push    ax                      ;Save AX and ES
            push    es
            mov     ax,40h                  ;Point ES to the BIOS
            mov     es,ax                   ;  Data Area
            mov     al,es:[17h]             ;Get keyboard flags
            and     al,03h                  ;Zero the upper 6 bits
            cmp     al,03h                  ;Are the Shift keys pressed?
            pop     es                      ;Restore AX and ES
            pop     ax
            jne     kb2                     ;Branch if they're not
            call    disable_video           ;Blank the screen
            iret                            ;Return from interrupt
kb2:        push    cs:[time]               ;Reset countdown timer
            pop     cs:[count]
            cmp     cs:[blanked],0          ;Is the screen blanked?
            je      kb_exit                 ;If not, then exit
            call    enable_video            ;Unblank the screen
kb_exit:    iret                            ;Return from interrupt
kb_int      endp
And here is the code to hook the interrupt - this runs at the beginning of the program
            mov     ax,3509h                ;Hook interrupt 09H
            int     21h
            mov     word ptr int09h,bx
            mov     word ptr int09h[2],es
            mov     ax,2509h
            mov     dx,offset kb_int
            int     21h
The entire program is too long to post here - 33KB. But then, you only want to see an example of what to do....
Here's another example which checks for various alt/ctrl/key functions
even
New_09     proc   far
           sti
           pushf
           push   ax
           mov    ah, 2                 ; get shift key status
           int    16h
           and    al, 0Fh
           cmp    al, 12                ; alt/ctrl?
           jne    @@0                   ; no
           in     al, 60h
           cmp    al, 19                ; 'R'?
           je     @@1                   ; yes
           cmp    al, 31                ; 'S'
           je     @@1                   ; yes
           cmp    al, 16                ; 'Q'
           je     @@1                   ; yes
@@0:       pop    ax                    ; exit if not my hotkey
           popf
           jmp    cs:old_09

@@1:       push   bp
           mov    bp, ax                ; save scan code
           in     al, 61h               ; reset keyboard
           mov    ah, al
           or     al, 80h
           out    61h, al
           mov    al, ah
           out    61h, al
           cli
           mov    al, 20h
           out    20h, al
           sti

           mov    ax, bp                ; restore scan code
           cmp    al, 16                ; was it Q?
           jne    @@GetMode
I don't remember now why all the ins and outs were needed (this code is from 14/05/1992 - twenty years ago!!!!).