All you need is a good tubing cutter
(Note that a tubing cutter is different from
the heavier duty pipe cutter
My tubing cutter is made by Ridgid
and it's one of my favorite tools,
very well made,
with a ratchet for quick advance
and a spring for quick release.
(A little while ago,
I couldn't help myself and did the very guy thing
of getting a second, smaller one,
just to have,
when it caught my eye at Home Depot
Advance the cutting wheel
(using the ratchet feature if your cutter has this)
until it just touches the pipe.
Double check that you're lined up with your cutting mark.
Then begin rotating the cutter around the pipe.
With each revolution around the pipe,
tighten the knob which advances the cutting wheel
It'll take 10-15 revolutions
before the pipe is cut fully through,
but they'll be 10-15 easy revolutions.
It's possible to advance the cutting wheel further with each revolution,
making for fewer revolutions in total,
but you'll work harder,
and the cut ends of the pipe will be significantly rounded over,
requiring a lot of reaming lest they impede the eventual water flow.
The last revolution or two will feel easier,
and on the last one you'll go all the way through,
and suddenly the pipe will fall cleanly in two.
(Don't drop your cutter!)
If you've taken the conservative approach,
advancing the wheel just a smidge per revolution,
you'll have a very clean cut
with just a sliver of deformed metal
impinging on the inner diameter of the pipe
around the circumference of the cut.
You can remove this either with
the reaming blade built into the tubing cutter,
or a separate reamer,
or just a rat-tail file.
And that's it!
Double-check the fit,
and you're ready to start soldering!