What he said! Let me expand a little.
Learning when to water is a knack that you just have to pick up on. "Overwatering" is a term that we hear often, and all too often it is misunderstood. Overwatering isn't about the "amount" of water - it's about the "frequency". Cannabis really does prefer "wet and dry" cycles. In other words - "Soak it good" (when it needs water), then "Let it get good and thirsty" (before you water again).
Once you get the hang of it, it's like riding a bicycle, but until then let's stick with "Plan - A" and "Plan - B".
Plan - A "Soak it good"
Start with a plant that "needs water", the surface will be dry and may have formed a sort of "skin" or light crust. That's ok, you can either brake it up or leave it alone - it really doesn't matter! The point is that you have to wet the whole surface evenly. I like to use a spray bottle with a medium-fine nozzle adjustment for this purpose. Spritz the surface evenly and well, it should soak in quickly, if it doesn't you need the grower's secret weapon "Liquid Dish Soap"(more on that later). If the water soaks right in, quickly, you don't really need to spray (I rather enjoy it myself!). Next comes the dreaded "Turkey Baster", I use this to deposit lots of little drops, spread out an inch or so apart over the whole surface. Each little drop forms it's own "puddle" and they should all soak in evenly. The reason for making sure the entire surface is evenly wetted is to avoid forming any "Dry Pockets" in the soil. If you have "several" plants, go around and do each step to all your plants before doing the next step. If you only have one or a couple of plants, wait a little bit between each step - that gives the water a chance to soak in, evenly! Keep adding water, a little at a time, slowly. The whole point here is to avoid letting the water make a "channel" that it just runs through, leaving isolated dry areas in the soil. After "a few" applications the pot should be thouroughly wetted and "Peeing" out through the drain holes (all the drain holes equally, not just one or two!). Ok, at this point the pot should be truely saturated -pick it up, feel how heavy it is? Get used to it! That "heavyness" is water!
Plan B "Let it get good and thirsty"
Ok, now you have a well watered plant. The first thing you will learn is that small pots dry out faster than big ones, plant or no plant! Get familiar with your plants! Pick them up and look close at them. Notice that as time passes, the pots get lighter - become aware of that process, get used to it! At some point the pot will get so light that you think "Surely the soil must be dry as a bone!" - that is probably a good time to water it again!
A good way to find out is to let the drying out process go a bit too far. On a day to day basis you "feel" the pot getting lighter, at the same time you are watching the plant for signs of "I'm thirsty!". When the plant first starts to droop a little, you know you have gone too far! Do this as a "One Time" experiment and your plant will be fine - but you know to water it before it gets this dry again!
Remember this, too much water just fills all the "Voids" in the soil and makes it impossible for oxygen to get down to the roots, that's a bad thing! A periodic drying, followed by an even saturation, causes the oxygen to get sucked down into the soil - as the water drains through it pulls fresh air behind it.
If your soil resists taking water evenly (if it "beads up" on the surface in some places) you need a "wetting agent". 2 or 3 drops of liquid dish soap in a quart (or litre) of water should be enough (it doesn't hurt the plant - just don't over do it!). And only use soap as often as needed.
This same mixture can be used to eliminate dry pockets. Put your pot inside another similar, empty, pot that has the drain holes blocked. Now, slowly, add the water (with a few drops of detergent per quart) until both pots are flooded. If any bubbles come up through the soil, wait for the bubbles to stop forming. Immediatly, remove the pot with soil in it. Let it drain well and let it stand a few minutes. Then, gently, replace the "plant pot" into the pot with the water in it - so that the drain holes just barely contact the water. Don't force the pot down into the water, just let it float and sink at it's own rate! This makes any dry pockets raise to the surface as "bubbles".
