301 is similar to 501. The only difference is that the player must score a double to start subtracting points from the starting score of 301. Any double can be scored to start the game, including the double bullseye.
When starting the game, only the double score and any successive dart(s) in that turn count for score. There is no "busting" of the double in; darts that hit inside the double ring simply do not count, unless they are thrown after the double score.
When the double in is scored, it is scored at the full value of the dart. If the double 20 is hit, then the player scored 40 points, plus any additional points scored after the double score.