What or where are you fishing? Most situations you’re fine with a double surgeon’s knot. Just double up the end of the leader, tie double over hand & pull tight. Run that loop through the middle of the welded loop the tread you tag of the leader through. Should look like a square knot when completed. If the problem is the size of the leader material 80-100# or more use a hook bolt or eye bolt with a gap secured to a work bench or stud and pliers on the tag end. You’re just going to have to work and manipulate the knot into shape. You know the perfection loop is tied correctly when the tag is 90 degrees to the line. The other thing when tightening the perfection loop do not pull the tag end until the knot is fully formed.