OK, I will start to put the details in; but let me start by saying that neither skill level nor ca$h should ever stand in the way of your hobby. So with that in mind, this will be a quick tutorial with a few pictures included to show you a type of objective marker that just about anyone could build (for almost any race). And let me add that I will make this piece of terrain using whatever I find available (no bitz, nothing purchased). In other words this will be a "Cheapy Cheapy - Easy Easy" project (which is good because I am not that talanted, and I am pretty broke at present).
1... Look around the race Codex and gather a few ideas of what the race is all about, or what your army has for congruity, etc. In my case I really liked the idea that Necrons have little obilisk's that often look like crystal or stone. In fact I even use bits of cut up sprue on all of my bases to incorperate the effect or "Little Crystals growing from the natural suroundings as my Necrons appear". Here take a look

Thus I will incorperate the Little Crystal Obilisk into my project for
Mission Objectives and simply make a bigger crystal on a bigger base all by itself.
2... Now I need to gather some materials. Cardboard for the base as I had an old box laying around, some paper because the printer is beside me, School Glue as you always need some adhessive of some sort, oh look some foam was in the cardboard box so I will use that as the start point.
3... Now gather your ideas into the actual project. I will cut out a little Crystal shaped chummy and glue it to the base and then paint it up to match my army and style (easy enough I think). The foam is soft and crappy and paint doesn't stick to well to it, so I will glue a layer of paper over the foam to help form it, stiffen it up, and give me a better painting surface. I will flock the base with a various assortment of static Grass, snow flock, and sand because that is what I happen to have - but laundry detergent (powder) would work just as well, and yes I have used it in the past. Here is where this took me.
4... Add some Race Appropriate details either with paint, with stuff like bits, or make your own using plastic spoons (work well for Eldar, etc), or Green Stuff or some other modeling clay like stuff. I will use Green Stuff becuase I happen to have, and I find it easy to sculp, cast, etc.
5... Paint it all up, call it done, and throw it directly into service - or build a few more so that you can do multi-mission objective based games, etc.
I did mine to match The Mighty Green Machine paint scheme; but you should do your own appropriate to your army. For me its Black primer, followed by progressive edging out with Dark Angels Green, Snot Green, Goblin Green and Scorpion Green. Followed by a wash with thinned thrakka wash, and then a layer of Ardcoat to make it shiney. The reds were done with Scab red, Blood Red, and Orange. Simple and effective just like most things Necron in nature.

Here is what it looks like with some of my army around it

Comments, criticisms, ideas, etc are always welcome.
Most Importantly though, Have Fun.
Cheers
SupREME-10
Necrontry Lord
TMGM Necron Cell 100111