The end of the pipe must first be cut square. Wrap a piece of heavy paper around it tightly, aligning the edge of the paper. Scribe a line around the pipe and cut, grind or file to that line. If possible drill a hole in the plate centered on the pipe and run threaded rod through the plate and pipe to clamp it for welding. Tack weld on opposing sides in several places and weld in segments on opposing sides to equalize warpage.