To progress into doing the front lever, I recommend starting with isometric holds. (onnit.com)
Once you can do the 3 isometric holds for 60 seconds each increase the exercise difficulty by either decreasing the rest time between sets or making the transition to negative one arm pull ups. (unique-bodyweight-exercises.com)
The program uses, on each exercise, 2 of the 3 types of isometric holds as defined by Todd Kuslikis in his excellent, recently published e-book:: «Isometric Strength» Those types are: (ashotofadrenaline.net)