Tape the backerboard with the fiberglass mesh tape and some thinset. Let it dry before you tile and thinset again.
[ame]http://www.youtube.com/watch?v=IHgIB7la72s[/ame]
Also I would use some redguard or a similar waterproofing over the concrete board, under the tile..only if this is in a shower.
[ame]http://www.youtube.com/watch?v=DjsEaeaQwV8[/ame]