It's possible as I've done it, however it was years and years ago and I do not vaguely remember how I did it. But if you've found instructions, either they will work or you can make it work if I did.
If I remember correctly you needed tv out? It actually worked pretty well considering, but I'd recommend you just buy a proper gun if you're that bothered about it. I just did it out of interest.
ADD: Looking at the link, I'm fairly certain those were the same 'plans' I used. (Svideo)