Wood ash contains other soluble salts in addition to potash. Adding wood ash to the soil will increase the pH. Does the soil test allow for an increased pH?

As for how to make potash from wood ash, it must be on the web somewhere.

An application of a nitrogen fertilizer with potassium such as 10-0-10 should be sufficient for a few years as far as potash requirements are concerned.

If you have a wood burning fireplace, just take the ash from there as your potash. It's that simple.

If you burn wood you produce a lot of air pollution that may not be necessary. Wood contains lots of other nutrients that can be lost by that burning too. Use that wood as a mulch and while it will take longer to have those nutrients released you contribute less pollution.