The buttercream should be on the thin side. You can try to smooth it from the back (before freezing) or pat it down to reduce the wormy effect. YOu can also try to smooth it with viva (if it is a crusting icing) after you have applied it. I am not sure if that is what you mean by bumpy or not. You can do a search for squirrellycakes fbct recipe. It is a perfect consistency for FBCT, but just so you know it does not crust.
Otherwise, I am not sure if you mean like "wavy" from the wax or parchment paper? Than happens sometimes. I am not sure what to do about that.
This is what I do for my FBCT--
tape the design to a cutting board
tape waxed paper over that
outline in black with #0 or #1 tip
color in the lines, don't worry if you go over the lines too much
when you are done coloring in and outlining, fill a bag with the icing you are using to ice the cake and pipe it over top of the finished picture at least 1/4 inch or so), then take another piece of waxed paper over the last coat of icing. (remember this is the back) and smooth it gently. I freeze it, then after it's frozen it should be good to go! Here's the last one I did so you can see it.
I agree with everything sharitos says, though my FBCT's don't turn out as great !!! I don't know if you wan't to hear this, but I have found better luck with chocolate transfers. I only have fbct's on my photos page but I find chocolate is a lot easier to use. Do it the same way as the fbct, but I have found that you don't need to smooth it out. Just my thoughts !! 
Hopefully, you return to this forum Mizsheli!
My second BCFT did not turn out either! i'm so sad. The outlines were smudged. What kind of BC do you use and do you freeze the outline before you fill in the resr?
I use crusting BC, and I don't freeze it until it is completely filled in. I do some pretty intricate ones sometimes and the BC always seems to set up before I get the outlines done, so maybe that's why my lines don't smudge....I also 'flood' the coloring areas without touching the outline at all, cuz it doesn't matter what it looks like on the back. Can you scan a picture for me so I can maybe tell what happened? Or did you throw that one away too?
When you say bumpy, are you meaning that after you have put it on the cake and after it has defrosted, that it looks "hilly"? if it doesn't it means you have uneven layers of buttercream on your FBCT. It's really important to make it evenly coated. I tend to make my transfers on the thin side (never more than a 1/4 inch thick) and I do not put a backing on mine either unless I see that it didn't fill in well.
I have used the Wilton crusting buttercream (all crisco), 1/2 crisco 1/2 butter and all butter - they all work fine... just make sure your icing is on the thin side.
For worming, I deal with that after the cake has come to room temperature and the top has crusted. I then use the high density foam roller and roll away the worming - works great!
With that said, I too find (with my one experience anyway) chocolate transfers to be easier - especailly if you buy precolored candies - no tips or anything... but... it's not the same. FBCT are fully integrated with the cake and you can cut into them and all is good. With chocolate transfers, you have to remove them to cut the cake and they don't look "at one" with the cake, but sometimes you want something 3d and then the chocolate transfers are great!
Melissa
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