If you still have the retaining hardware consisting of a bolt and a cap, put them in all the way but not tight. Then back the bolt out against the cap...................the bolt acts as a jackscrew.......it is threaded into the spindle, the cap is threaded into the crank, essentially the bolt being backed out against the cap pushes the crankarm out

That said I have had the cap get pushed out of a jammed crankarm.................so be careful