How do I correct not withholding funds from an ira withdrawl

I had to pull my funds from a simple IRA and had planned to have them withhold 35% (10% penalty & 25% income tax) but then he asked me how much I wanted to withhold for state.  I hadn't researched how much I would have needed to withheld and after speaking with him, he assured me that I could just hold onto the portion I thought I would owe and pay during next tax season. Now at home reading up and I'm seeing that there could be an underpayment penalty added on as well. How do I go about making an estimated tax payment to the irs so that I don't acquire any additional penalties? The amount I pulled out will not put me into the next tax bracket.  I've never owed money before, always have received a tax refund. I saw some stipulations of lesser value of 90% this year or 100% of last year.  Didn't quite understand that.