You'll need a spare yoke and stepup transformers to match impedance. Set the
original yoke aside still hooked up and put the spare on the tube. To use a
computer monitor you may need to have an old PC just to get it to turn on.

In fact, to set the purity you'll need to feed the monitor all one color from a
PC, then you can interrupt the video signal and start playing around with that
too, use a low value cap fo example to feed the high frequencies to modulate
the beams.

(yes, as you suspect I have done it, on an old 21" round color TV)

Have fun.

JURB