Around my office, the walking trails are dg. They just lay and compact it on the soil with no underlayment. I see them refresh it about once every year. Maybe the heavy foot traffic keeps the weeds down, I never see any. I work for a very 'green' company so doubt that herbicides are used although I have no confirmation of that.

You are planning it for mulch around your plants? I've thought that it would look nice in my cactus garden.

Yes, I thought it would look better than gravel in beds around the house.

I haven't yet used dg but have researched this in preparation for putting in some paths. So I'll share that with you.
First off, you WILL have to contend with some weeds which naturally land on top of it and take root. This is much more common than weeds sprouting from beneath I'm told. However most people say weeds are really not a big deal if you're somewhat diligent in removing those that pop up. You can use an organic spray (such as the vinegar mix I've posted about and others have mentioned). Just keep it in a handy spray bottle and problem solved. A couple of times a year (spring and fall) ought to be enough.
I've also been told it's best NOT to put down weed cloth because you want your first layer of gravel to embed into the soil. And besides, as I've mentioned, the weeds are much more likely to blow in on top rather than coming up from below.
The first step is to clear the ground of most weeds/grass however you choose to do it and then get it as level as possible. If you should decide to use a tiller then till very superficially - better than deep tilling I'm told because that would just spread and activate dormant seeds. You could also cover the area with plastic or cardboard until the grass dies down.
It was suggested to me that the entire height of the gravel be about 4 inches and that you only need to use the more expensive dg for the top inch or so. Using a road base gravel is best for underneath it. I know a gravel company near me calls it "dirty-d". It's used for roads because it forms a thick mat and holds (it has angular crushed limestone).
Lastly, it was recommended that I compact each layer (a layer being one or two inches thick) as it's put down. That assures that it is well integrated and compacted and won't move around. You can either rent a fairly heavy hand rolling machine or hire someone to do it. So this is what I plan to do. Others may have alternative ways, but this was what I was told by some professional landscapers. Good luck!

Thanks mrs. wiggley. I've just about decided not to go with decomposed granite, mainly because of my wood floors. I know dh would walk right in with it embedded in his shoes. I guess I'll stick with the mulch even though it doesn't look as good to me.

I like the look of DG but it tends to get washed out rather too easily during heavy rainfall if they are not large enough to stay in place.

Another name for dg is roadbed. That's what its used for most of the time. It does compact and stop weeds. I used it under the brick floor in my greenhouse. I wanted a porous floor that would stay level and its working great. Its been down for four years and there has been no shifting or sagging of the brick. Around your cactus a better mulch would be pine bark. You'd have to replace it about every 3-4 years but it breaks down into a nice compost and enriches the soil. Its also lightweight so if you get it on your shoes most of it will fall off before you get back to the house. Dg would look ok for a while but eventually will sink into the soil and disappear.