If you're confused about neutral and ground, perhaps it's the welder has a NEMA 10 connector. (Hot hot neutral). This was the standard dryer plug in the old days before safety was important.
For some strange reason a lot of people still use NEMA 10 plugs on welders.
Anyway, if your welder has a NEMA 10 plug, change it to a NEMA 6 plug (Hot-hot-ground) which is on the same shelf 2 bins over. Welders need ground if you don't like being dead.
Now it is a simple matter of getting “cordage” which is special flexible cable made for daily flexing - well you know, every one of your home appliances has cordage.
You need 3-wire, which will be black white green wire colors. Here's the important part: mark the white wire red on both ends, It will not be used as neutral, but rather as second hot.
Now, it wires up simply. On the NEMA 14-30 plug, you have 4 pins and 3 wires. Don't connect anything to neutral.