What I found most challenging in learning to navigate was flying the aircraft subconsciously.
Up 'till then all the flying was done consciously. Turning, climbing, maintaining a heading, altitude, airspeed, doing checks and looking for other traffic: everything was done with 100% focus. When starting nav, you have to do all those things continuously but subconsciously because your mind is busy elsewhere.
So instead of focusing on the altimeter you put the aircraft into a climb attitude with the correct speed, trim it, check everything is alright, make a mental note of your vertical speed and focus on something else. Every 30 seconds you glance back at the altimeter and VSI to see if everything is going as planned and only a few seconds before leveling off becomes leveling off first on your mind again. In level flight, you trim the aircraft and then it's just a glance at the altimeter every now and then to see if you're still there.
Same for heading changes. Instead of ticking off the degrees on the DI you roll the aircraft into an appropriate bank, check your progress every 10 seconds, and only consciously look at the DI again when the aircraft is more or less on the appropriate heading, to fine-tune the heading.
It's challenging, but lots of fun!