Tablet Weaving

Tablet weaving uses rotating cards to change the positions of warp threads. If all the cards are rotated together in a particular direction each time the weft is passed through then a characteristic diagonal pattern develops. A pattern which has the same number of rotations in both forward and backward directions overall will lie flatter.
More complex patterns can be made by rotating individual threads separately.

For detailed instructions see Phiala's String Page, Jenny's Braids, Bands and Cords Page, Intermediate Tablet Weaving: Figured Double-Face Weave, A Saxon Threaded-In Tablet Weave, Viking-Style Tablet Weaving: Birka Strapwork Motif or The Vikings' Basic Kit Guide.
"The Techniques of Tablet Weaving" - Peter Collingwood - Random House - 1566590558 is a useful book on the subject.
There is also The Ultimate Tablet Weaving Bibliography and an analysis of archeological evidence at The textiles in the Oseberg ship.