
Here are the procedure notes:
http://www.bva.co.uk/uploadedFiles/Content/Canine_Health_Schemes/hip-dysplasia-scheme-procedure-notes-july-2015.pdf"4.2 Positioning
A standard position for radiography must be employed as follows:
(a) the dog should be placed on its back with the pelvis over the middle of the cassette or detector and the X-ray beam centred on the midline between the hips (i.e. the centring point should be at the level of the cranial edge of the pubis, by palpation).
(b) in order to avoid lateral rotation, the body should be supported in a straight line using a cradle or sandbags. The thorax must be upright and symmetrical since tilting of any part of the dog's body is likely also to cause tilting of the pelvis and asymmetry in the appearance of the hips. Lateral tilting of the pelvis can be recognised as a disparity in appearance between the ilial wings and obturator foramina on the two sides. It may be corrected by raising the pelvis slightly on the side on which the ilial wing appears wider and the obturator foramen narrower on the radiograph.
(c) the hind legs should be fully extended and adducted so that the femora lie as near parallel to each other as possible: they must not be over-adducted (i.e. they must not converge towards the stifles).
(d) the legs should be inwardly rotated so that the patellae lie centrally in the femoral trochlear grooves i.e. the stifles are upright.
(e) suitable ties or tape placed around the distal femora or stifles should be used to achieve correct adduction and inward rotation; ties must not be placed around the proximal femora, pelvis or or hips and radiographs showing restraint at this level will be rejected.
(f) poor positioning which allows either lateral or longitudinal tilt of the pelvis or incorrect positioning of the femora may prevent accurate radiological assessment of the hips; such radiographs may be rejected."
Here is how the scoring relates to the physical parameters and has pictures of how a dog is laid for the x-ray:
http://www.bva.co.uk/uploadedFiles/Content/Canine_Health_Schemes/Interpretation-and-_use-of-BVA-KC_Hip-Scores(1).pdf