This pattern is now available on Ravelry – http://www.ravelry.com/patterns/library/celtic-cross-bookmark
I do not consider this to be a particularly difficult thing to make but I would also not consider that this is a pattern for beginners.
The pattern includes all four versions pictured above.
The pattern itself is given in an abbreviated form and there is an explanation of the abbreviations in both UK and US terms.
All four versions of this pattern create a long strip that can be knotted by folding over and under to make a cross. Held in place by a separate circle of crochet.
The first two crosses are suitable for bookmarks, the third is the same size but a bit thicker, so maybe less suitable as a bookmark but would make a good gift. The fourth is a larger cross that could be hung on a wall.
The first three bookmarks worked in #10 crochet cotton come out about 4 inches (10cm).