Probably one of the easiest bits of terrain to make; altars can range from a simple stone to a large monument of glory however all altars are made to emphasize something of importance.
The stepped altar:

A stepped altar; probably the seconded easiest altar I know to make and easily the most recognisable involves two or more stepped layers leading up to the figurehead (a pedestal, statue, etc.) as well as one or more sets of stairs.
Almost any material is suitable for making these kind of altars however for this example I've used 3mm thick MDF and 1mm thick Plasticard tile sheets.

The perfect step:

When it comes to stairs it can be tricky to make a set that a model can balance on; too steep or deep and a model will topple easily. A good depth is about 4-5mm like wise a length of 10mm and width of 25mm will give a nice natural slope that a warhammer model can easily balance on
How to make wood look like rock:
The easiest way I know to get a rocky finish is to scrap on some concrete plaster finish with a firm strip of cardboard (It works very well even when passed it's use by date; the stuff I'm using expired back in 89

)

The pinnacle piece:

As you may have noticed I have cut out two tiles just off center of the top layer and left it blank, this is intentional; the gap created will serve as a lock in point for the center piece when I have made one. Thus I can change the center piece to suit the occasion. Will it be a statue of a spacemarine or a holy sword of the empire? An Eldar world spirit or an Elven place of worship? A monument to chaos or ah... well.. a monument to chaos?

Well thats it for now yeah I know it's not much of a tutorial but I will expand it at a later point.
On a side note just because 'I' thought it was funny when painting this altar I thought of using some left over water down fenris grey instead of a wash however it was thicker than I remembered and it started to turn into a mess. In a fury I topped up the paint with the brush cleaning water then splashed it all over model (If it's going to be blue, it's going to be blue right?) any way I tipped it on it's side and noticed that the paint clung to one edge leaving a nice weathered shadow. I guess sometimes you just fluke things.