Measure the ceiling along the intended beam line. Clamp one length of wood in your Workmate. Clamp another overlapping length of wood alongside it so the combined length is equal to the length of the intended beam.
Saw through both lengths of wood together in preparation for making a scarf joint. Cut at 45 degrees or less, according to the beam dimensions, to maximize the joining surface. Join the two lengths together with wood glue, clamp in the Workmate and leave to dry.
Select two wood dowel pins suitable for reinforcing the scarf joint. Choose a drill bit size according to the dowel pins. Drill through the intended top surface, i.e., the surface that will be fixed to the ceiling, at a point that will meet the diagonal saw cut halfway along its length to the depth of the dowel pins.
Squirt wood glue into the two holes. Insert the dowel pins. Wipe off displaced glue and allow to dry.
Stain and varnish your beam using oak stained varnish. Allow to dry. Your beam is now ready to be fixed in place.