Solenoid Valve Wiring

Easy. Get a length of flexible cord with black/white/green. White goes to large flat prong, black to small flat prong, green to round prong.

At the other end, black to one red wire, white to the other red wire, and the green (ground) to the frame of the solinoid.

I suspect that the best splicing/connection bet is to use the right size of crimp-on sleeves. Check the size of the wire involved. Probably the blue ones, or maybe the yellow. Those sleeves (you can buy an assortment) and the matching crimp/stripping tool are something you should have around anyways.

No need for soldering.

Maybe a wire tie somewhere to anchor the cable to the side of the solinoid to take the strain off things. Wire ties are also pretty handy to have around as well.

All this in the electrical section at Lowes or Home Depot.

If you want a switch in the cord, get an in-line switch for the right sized cord and switch the black wire.

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To make the point explicit for OP, being as it's an AC-powered solenoid the reason there are two undifferentiated power leads is that polarity is immaterial...either way is fine.