Play practice mode 3 or 4 times. Yeah, that sounds simple but you need to know how hard level actually is.

You need to set daily % goal. For example - 10% per day. So you should be able to complete most of the levels in just 1 or 2 weeks.

When you are training new part of level you can create copy of the level and then set start position.
Here is breakdown:

• Create a copy of level
  https://images.steamusercontent.com/ugc/16144493807928844078/7FE81531404514E480E4B227E6A8378DA72F1B22/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false
  If level has password, then go to the comment section and sort comments by likes. Most liked comments usually can tell you password.
  
  
• Set start position wherever you like
  https://images.steamusercontent.com/ugc/12945162484029593575/2B1CE2127014DE5B6AA026D31AD92E218968A97F/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Hope I helped you. Good luck in beating level!