Totally agreed that tripod is a must. I find that for 50mm above, tripod is the key.
Very easy just switch to Live view, next turn the apperture ring to your liking, then turn the focusing ring to focus. You will know when you have focus. Then set the exposure compensation, lock the exposure lock. And then you can start to record. Remember to double press on the info button, you will be able to assign AE-L/AF-L button to AE lock (hold). This will help you lock the exposure, the cam will not auto adjust during recording.
I think the 135mm f2.8 would be really great for concert or performance, the focus length seem just nice for a range of 15 to 20m.