thanks in advance

The original source is the paper "The reinforcement of slabs in accordance with a pre-determined field of moments", RH Wood, 'Concrete', February 1968, and a follow-on letter from GST Armer, 'Concrete', April 1968. Their method is summarised in the book 'Concrete Bridge Design to BS 5400' by LA Clarke.

Wood Armer is usable only for design, and only where the reinforcement design is relatively straightforward (e.g. ultimate limit state analysis) - it can't optimise for designs where the relationship between reinforcement and load effect is non-linear (as is the case for crack-width limits at the serviceability limit state, for example).

Wood Armer is useful because it allows moment triads from grillages (Mx, My, Mxy) to be transformed into simple bending moments in two directions for reinforcement design. This is important because the twisting moment Mxy can be significant.

An alternative to Wood Armer is the method of Denton and Burgoyne, see 'The assessment of reinforced concrete slabs', SR Denton and CJ Burgoyne, 'The Structural Engineer', Volume 74, Number 9, May 1996. This is useful where the reinforcement is already known e.g. in a existing bridge slab.

Modern software can automate the Wood Armer or Denton Burgoyne calculations, for example, SAM-LEAP5 is a program I use which can convert the grillage moments automatically.