Bev, practice on some smaller sandwiches - say, 10" - 12" square. With McTavishing, there usually isn't any marking because it's a background filler, but you can certainly mark off sections (that'll make it easier than just looking at that big old blank background!)
Bottom Line (the 60 wt thread) is normally used in the bobbin, so you would use it in both for McTavishing. Here is the Superior website where you can find out all about them:
http://www.superiorthreads.com/shop/...ools/products/
Remember, its all a matter of practice, and yours won't look like anyone else's - it's not supposed to :D