I'm not sure. I would think to be as realistic as possible, you would want the indentations like real leather would have, and paint in those depressions. There are some things I will flat out tell someone, even if they say "but I haaaaaate fondant"....this must be done in fondant unless you want a cakewreck.
very good crusting BC fully crusted
then you'll have to use a dull object (like one of the tools used to make gumpaste flowers -- I'd probably choose the smallest ball tool) (even handle end of a big artists brush)
gently trace into the BC, just enough to dent it.
slow & tedious
----
now if only there were a way to print the design on Viva paper towel -- could lay that one and trace through it to be even surer of not cutting in too deeply.
Hi, I have used this on fondant and would work ok on good crusting butter cream. In the fabric/home decorating area of a local hobby store look for a fake leather look material with the tooled look you want. Lay a piece of plastic wrap down over the top then the material. Use a fondant smoother to impress the pattern (deep grooves and patterning) into the crusted butter cream, gently remove the fabric and plastic wrap, then air brush! I do it on fondant, so would work if you apply the correct pressure on the butter cream. Good Luck.
Hi
Maybe i'm being thick here but is it the swirly pattern you're after? If so i have a suggestion - not sure if it will work coz i have never used buttercream( i'm a fondant person) but i'll let you be the judge of that. In one of my books to help people like me who can't use a piping bag (oh the shame! 
) the name/pattern is piped onto perspex with RI, once this dries the perspex is turned over and placed on the cake to imprint the pattern/name or whatever. Like i said i have no idea if this will work on buttercream just thought i'd suggest it in the hope that it may help. 
Hey, that's MY cake! I'm glad ya'll like it so much, haha.
It's all fondant, except for the barbed wire and piping on bandana- rare for me since most my stuff is usually all bc.
One of the reasons I decided to use fondant was because I knew I could make impressions in it, then rub on/rub off a darker color to get this effect. I loved the "antiqued" look it created.
To do it out of buttercream, I would ice in the lighter brown color then pipe with a #1 tip (or smaller!) in darker brown in the pattern you want. Then VIVA the whole thing, being very gentle. It will smoosh down the piping into the light brown and kind of give you that look.
Just remember with buttercream that you'll lose the shading and depth of the rubbed on/rubbed off color creates. Unless you airbrush too, but if you're going to go to all the trouble you might as well use fondant!
Hi
Maybe i'm being thick here but is it the swirly pattern you're after? If so i have a suggestion - not sure if it will work coz i have never used buttercream( i'm a fondant person) but i'll let you be the judge of that. In one of my books to help people like me who can't use a piping bag (oh the shame!
) the name/pattern is piped onto perspex with RI, once this dries the perspex is turned over and placed on the cake to imprint the pattern/name or whatever. Like i said i have no idea if this will work on buttercream just thought i'd suggest it in the hope that it may help.
I was thinking the same thing. If indeed all you need is the pattern indented onto your bc then just use an RI stamp. You can pipe the RI onto the plexiglass and use the plexiglass as a stamp once the RI dries.
There is a tutorial with pictures in the forums (if memory serves it was done by rocketgirl) but I can't find the search option right now (must be a slight hick up on cc) otherwise I'd post it for you. I'll check again later to see if the search feature is back and then I'll add the link.
Looks like it was a temp gliche.
Here's the link:
http://cakecentral.com/cake-decorating-ftopict-609974-stamp.html
HTH
thanks again, these are great ideas! I know using fondant would be more practical but I am determined to make BC work. I guess I am just a BC kinda gal. I mainly use fondant for accents. No one in my family likes to eat the fondant cakes, even the MMF.
There is a pic in my photos of the finished cake. It was to big to attach to post here. I think it turned out great. I used the end of a small paintbrush to make the tooling marks then airbrushed them dark brown. I used a papertowel that had a stripe pattern on the outside for texture.
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