Polar armies would look sort of lame if the Ice Chariot was sledding across ... grass and rocks.  To give the right feel, I based them up with the following technique.

1. First, look at your local home improvement store for the clear plastic shields used with industrial/workplace overhead florescent light fixtures.  There will be several styles.  At Home Depot one of them is this sort of irregular cracked ice look.  If there are any broken ones, you can often get the store to sell it to you for craft purposes for much cheaper -- you don't need much.  But they aren't expensive anyway.

2. Break up the shield with snips of some type into irregular shapes.

3. Take MDF bases (already cut to size -- I do mine on my tablesaw with a very good blade, but some commercial dealers also make them for sale).  1/8" (3mm) is a good thickness.   Place some ice pieces on the bases (not all the bases -- about half of them) in random places and mark position with a Sharpie.

4. Paint the MDF black where it will be under the ice.

5. Glue the edge of the ice piece down with E6000 or other good glue that works on acrylic and wood.  Test first if you don't have E6000.  Don't spread glue across the whole surface, just do the edge.

6. Take paintable white bathroom caulk and spread it on the rest of the base, going over the edges of the ice to obscure the transition.  Use your finger.  Smoodge and tap the caulk to create texture and unevenness.  This takes a bit of practice.

7. Let the caulk dry.  Maybe two days.

8. Paint the edge of the base to match the ice (black) or snow (white).  See the Mammoth base above for an example.

You can either glue the figures to the base first (and smoodge the caulk around their feet) as I did with the Wolves and Ice Barbarians below, or you can glue the figure down with E6000 as I did with the Mammoth and the Ice Trolls.  Depends upon the figure.  And sometimes a mix, where I cut and grind the base off the figure so I can glue it directly to the ice.