For vertical jigging or pitching jigs I use 6-2 FireLine. You can use either Smoke or Flame Green for Vertical Jigging – use Flame Green for pitching (makes “line watching” much easier).
You will want to spool up your spinning reel about one third to half way full of mono backing (other advantage is your spool of FireLine will last for more reels). The backing “grips” the spool and it will never spin out. Then I just use a “figure 8” knot to splice them together. Take your two lines and put them end to end. Keeping both lines together, make a loop in them about 3 inches from the end. Twist the loop one time and put the tag ends through the loop. If you do it right, kind of looks like a figure 8 as you tighten it up. This knot should not be fished – if it starts appearing on the reel – time to re-spool.