call your broker or financial institution to find out how. there is no standard way among them.
the excess of the amount withdrawn over the amount contributed will be subject to income tax at ordinary income rates. The fact that this was capital gains in the a/c means nothing. you will get a 1099-R.
you don't say why you don't qualify but it may be possible, if you want, to convert the Roth to a traditional IRA. you would need to contact the same people for the procedure.
as to withholding can't say because that really depends on your 2020 tax situation.
