How to inject CO2 in to carboy to cover beer or wine?

I do not keg but would like to be able to put CO2 in my fermenter to cover beer or wine while conditioning.
This came up because I am conditioning 4 gallons of mead in a 5 gallon carboy.
But I think this might also be beneficial for beers that are long term fermented where head space is present.
Without any knowledge of CO2 or kegging, how can I do this?
THANKS

You could vent your primary into the secondary with a tube. I think Better Bottle has a nice diagram of it. I'll see if I can dig it up.

Found it. There's a lot of good info here that should steer you in the right direction. Here's a pic.

Chances are that when you transfer the mead it will off-gas somewhat, naturally filling the dead space with co2.

If your still concerned, add a touch of corn sugar. A quarter cup or so would fuel the yeast to add co2, without affecting the character of the mead. The only other way would be to buy a co2 canister and inject gas for a few seconds. Hope this helps

Boy, I hate to be a naysayer but I don't think just using co2 in over a gallon of headspace is a good idea, even if you did have a co2 tank.

The reason is that while co2 is protective, it doesn't hang around indefinitely and once you add it, it will immediately start disipating to reach equilibrium. Since meads are in the fermenter a LONG time, you'd have to do it often and even then I'm not sure it would stay for as long as you'd hope.

It'd be better to rack to a 3 gallon carboy and a 1 gallon jug, and top up to the bung so that any risk of oxidation is minimized. Using sulfites is also a good idea, as it will bind to the mead so that oxygen can't.

I have a kegging setup, but I still top up all of my wines, meads and ciders.

Another issue will be degassing when you want to bottle the mead. Much of it will off-gass, but you may risk having a gassy mead if you continue to flood with co2 and keep it airlocked.