Measure the placement of the flat carefully to get it centered between the die bar mandrel (slats) and bolt the package together securely. I suggest using a much thicker lead-in die bar to help keep the process oriented squarely while forming.
Pressing or pounding one end of this sandwich at a time was done to ease the forming process and is rather mickey mouse. It risks creating a misshaped product. Thicker die bars (slats) also help maintain orientation of the work being pressed in the die (angle iron assembly). However... the inner die bar slat(s) must accomodate the indentations already present in the flat or you may damage these indentations.
These days we have edge-folded flats available which are a great help obtaining a useful frame.