One of the steps to assembling your miniatures is to fill the gap usually left in bases after gluing the mini to it. We do this so that you're not trying to awkwardly glue stuff across a hole in your base and provide a surface to glue things like flock to.
There's a few ways to do this.
I've talked about using modelling putty to fill gaps before and this tends to be my preferred method, even for larger base gaps. Aside from Testors putty, I've even used autobody (car) filler. Fumes are stronger though, so I'd suggest sticking with hobby level stuff.
First off, I want to make sure I've completely filled that hole, so I run the putty along the hole, filling it to excess. (You can see all the extra putty sticking out of the hole.
Then you wait. Don't try to scrape or level the putty while it's drying.
Once the putty is hard, cut or scrape with a knife or file the excess putty. Make sure the bottom is smoothed, so the base sits on the table without wobbling. For the top, your first thought might be to leave it rough, because, y'know - terrain is rough. You're half right; What you don't want is a rough line down the middle of your base.
This CAN be done roughly and gives a nice rough terrain sort of feel to the base.
[UPDATE] Getting some great suggestions in the comments, so scroll down and take a look...