Location

Gas clouds are found wormholes and in K-Space in low sec and null sec. They are only found very rarely in high sec and then only via Exploration.

Skills needed

To mine a gas cloud you will need the skill - Gas Cloud Harvesting.
To learn the Gas Cloud Harvesting skill you need Mining at level 4.
You can install one gas cloud harvester per skill level.
The skill book currently runs 21.6M isk, and is available from all normal NPC sellers.

You will need a ship that has high slot turrets.
Unlike other mining, gas cloud mining can not be performed in any type of exhumer. (They can not mount high slot turrets.)
You will also need at least one Gas Cloud Harvester. You can have up to your max number of high slot turrets or your skill in gas cloud harvesting. (whichever is less)
A ship with gas harvesters attached appears to have nozzles coming from the body of the ship. The number of nozzles do not necessarialy match the number of gas harvesters fitted.

To be able to target a gas cloud you need it selected to display on your overview.
To do this, right click the overview, select open overview settings, select the filter tab, open the celestials folder, and check the Harvestable Clouds box.

Move within 1500m of the harvestable cloud (the range of a Gas Cloud Harvester 1 module).
And start your harvesters.

With a Gas Cloud Harvester 1 you will receive 10m3 (1 unit) of gas per 30 seconds (1 cycle).
At 10m3 each it does not take a lot of gas to fill most ships cargo bay.

Notice this is just for high sec gas and is not the same for low and null sec gas.
Gas from wormholes is a totally different animal and not used in booster production but instead is used in T3 manufacturing.

Gas from gas cloud sites are used in the manufacture of Boosters. (Other than gas from wormholes).
Gas from high sec exploration sites is of the Mykoserocin variety, these produce the basic synth boosters.
Synth boosters are NOT illegal and have no possible side effects.

Other gas from low and null sec make higher grades of boosters that are illegal and have "possible" side effects.
On these boosters you are guaranteed the good effect, but there are 4 "possible" bad side effects. (You may get none, you may get all 4, there is a percent chance for each side effect.)