I usually build mine using a slip roller (wrapping it around a post will work too), two plastic mallets, and a sandbag. At my age, think I should have an English Wheel and planishing hammer!!!!!

Anyway, start with a template made from posterboard, trace it on the steel sheet, cut it out and start fitting.... Not that technically difficult, but it is very time consuming to get a good looking one. (that you will cover with carpet anyhow!!!!) Got to make it presentable from the bottom side, too....