To get the best oblique, put the hand palm down on the cassette, then raise the lateral side (i.e thumb side) up so that the palm is not touching the cassette anymore but the tips of the fingers (including the thumb tip) and the ulna side of the hand are still on the cassette. Bend the hand at the MCP & IPJ to achieve this positioning.
The central ray is perpendicular to the 3rd metacarpophalangeal joint.
FFD is 100cm. Direct beam (no Bucky).
Exposure factors: depends on individual x-ray system. It should be around 40-50kV and 1-2mAs.
You may split the cassette into 2 to accommodate both projections in 1 film. A 10x12 cassette is ideal.