Same thing you would with a cake...
Put a little bit of icing on the board then lay the fondant over it.
Then cut off the excess fondant around the edge w/ a knife.
Then if you choose, decorate by either painting or crimping. 
I just did one. I put together three cake boards. For fondant to stick, you wet the board with water and lay fondant on board. Then let board dry for about 2 - 3 days.
Now you can cover the side on the cake board with fondant or trim off the excess and wrap a ribbon around the board.
Also, the cake you place on that fondant covered board is to place on a cake board the same size. To make that board stick to the fondant, I used royal icing.
Good luck
You will get the best results is if you have a thicker board, 1/2" foam core or drum. Then you can put a ribbon around the base for a nice finish.
Roll the fondant out a bit larger than the board. You need to add a bit of "glue" to the board surface. You can use a very thin smear of piping gel, corn syrup, even a bit of beaten egg white, or royal icing. You want just enough to make the surface sticky, not enough to make the fondant wet, cover the whole surface to adhere the fondant.
Place the rolled fondant on the board, smooth it, and trim the excess.
I used to use bc or piping gel to get the fondant to stick. But, in a Collette Peters book she said just wet the board then put your fondant on. I tried that on my last two cakes - Dora and purple electric guitar - and the water worked great. No more bc or piping gel for me.
I wet my board to make the MMF stick and then cover as I would a cake, trimming it and such. hth
I use my 1/2" foam board base and I wrap that in the Glad press n seal. I brush a little water on the top and sides of the wrapped board and then cover with the fondant (just like you would a cake). By wrapping my board first in the press and seal, when the cake is all gone, I can just peal it off and I still have use of my board. All the cakes in my photo area that have fondant covered boards, I have does this with.
Missey,
I like your tiarra, it does not look "busy" and too crowded with so many loops and curls. It's a clean design but very pretty...
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