adding a custom field in OsCommerce

Very simple job for the right person that knows OsCommerce software. All I need is an editable field.? I do custom embroidery, I built a webstore for a customer and when they pick thier product, their logo is automatically going to be embroidered.? Upon check out under the attributes section, I want to add a field that THEY can type in. Meaning.? For an extra $5.00 the customer can have their name embroidered on the right chest, so upon check out they can choose: Name Embroidery (no thank you) Name Embroidery (yes please) PLEASE TYPE NAME ______________? <I dont know how to add this custom field. The drop down for if they want it done or not is easy, but how do I add the additional field that they can type their name in?? Also in a perfect world, if they are NOT getting their name embroidered then the custom field does not have to be required, but if they DO want the embroidered option, I want the "custom field" to be required.