ok, I found the part I
threw away.
hope this pic explains it;
You have to get one of those biros with the cover grip.
Take the grip off. Take out the ink-refill.
Throw away the spring.
Find something to saw the barrel about 5 to 10mm less than the grip height. It doesn't have to be accurate. I guess if you can score the mark with a blade, you can snap it off, I don't know, I used a saw instead.
Cut the ink-refill with a knife or scissors, you'll probably have to bend it till it snaps off like I did and the ink won't spill right away, don't worry.
Insert the ink-refill and glue up the top (I just used silica gel). Make sure the tip doesn't poke out too much (like a real pen). Let it set.
Insert dummy pen into bottom of grip.
Insert manual battery into top of grip.
oh, I forgot one thing important,
* * Don't Use Auto-switched
batteries !!! * * (air pressure confuses them !)