I have 10-2 wire and a 30amp breaker. The wire has 4 wires in it,blk-red-wht-and grn.

The welder has 3 wires extending from it, blk-wht-and grn

How should this be done?

I'm guessing I could put the blk and what wire to the hot on the plug and grnd to grnd and bypass the neutral since there's no wire for it to connect the plug.

Then use the red and blk wires from breaker to the outlet along with the neutral and ground wires
Then

Put the red and black wires on the 240 breaker. White can be connected to neutral at the box and ground to ground.

At the welder plug, use black and red and green, where green is ground. Put a wirenut on the white.

On the plug of the welder, use black and white and attach to the red and black wires from the breaker respectively Green goes to green.

The welder wiring to the plug is only blk,what,and green. Im planning on using the white as hot/red as it would be the only way to get the 240 from the outlet.

Are you sure it's a 240volt welder?

What you have is a 10/3 cable. You can actually use a 10/2 cable.

If this is a straight 240 VAC unit and not a 120/240 VAC unit, then either red or white is not needed.

You would connect the Black conductor to the black on the unit and then use either the red or white wire connected to the white on the white.

The other end of the conductor would go to the second phase position on the breaker. If you choose the white conductor, then use either black electrical tape or a black sharpie to color the insulation at both ends so you know that it is a hot conductor. Also, make sure you cap off both ends of the ed conductor.