When I started working the planche again recently, I just played around with tuck planche holds for a couple of seconds (like YJ says). After a while, I felt like I was doing them for "time", so I started doing 5 second holds. Then, I just kept at it.
Also, I find the 60-seconds-of-total-work thing doesn't work well for me. I do better with 2-3 holds for time, but not straining until perhaps the last hold. For example, I'll do 3 x :08 second tuck planches, or maybe 3 x :15 flat back levers. But that's just me. Experiment and see what system works best for you.
I would highly recommend working on the back lever before starting into the unsupported-body planche progressions. There are many of the same muscles worked.
And if braindx posts, listen to every word he says. He really knows his gymnastics.
Some of my exercise beliefs:
Self movement is the prime athletic directive.
A person can gain as much strength as they'll ever need through bodyweight exercise.
There is more to strength than one-rep max.