How do I wire this 30A 250V straight blade pass and seymour receptacle?
Assuming you mean this 6-30R:



After you wire the green (ground, not neutral), it doesn't matter which of the two ungrounded (hot) wires are placed on the 2 'flat' blades.  240VAC (in the North American split single phase) is not polarized with respect to ground, because the neutral is not being used.

This connector can also be used with 2 hot connectors in a 3 phase system, which would then provide 208 VAC.  This is not common for residential use.

A Wikipedia article on NEMA connectors in general and NEMA 6 in particular may be helpful.