There are several ways that have proved popular and successful for my students. First, and possibly the easiest if you have brazing or welding facilities is to use a set of suspension fork crowns/yokes, either single or double. Buy some appropriate diameter steel tube to fit the crown bores, weld dropout plates to the bottom of each steel tube, the offset designed to give appropriate steering geometry and clamp them into the crown- like the orange one below.
Another method, as shown in the previous post is to extend an existing set of rigid forks using inserts and extension tubes like this one.
My personal preference, if you have the machining facilities is to manufacture a set of custom yokes and legs, these yokes can be made ornate like these on the vampire- the black un in the group pic
or plain as on my Drifter
This method has the advantage that you can design in the exact steering geometry that you want as well as the style that you want.
I had one student who integrated the fork legs and handlebars, welding the legs to the bottom yoke and making a split top yoke from ally billet
