Hand flying an arc and keeping your needle at 90 degrees the whole time is virtually impossible. To do that it would be best if you had no wind, and then you'd have to fly the airplane in a precise bank with an accuracy of about +/- 0.05 degrees throughout the whole arc. If you did have wind, you'd have to know exactly what bank angle you needed at all times, and of course be able to maintain it with great accuracy. For a human pilot, that doesn't work too well.
I first start my students off in the sim with no wind. Then I get them to intercept the arc. If you're flying perpendicular to the arc as you intercept (90 degrees), start the turn 1% of your groundspeed in NM from the arc. So if you're grounding 140 kts, start your turn at 1.4 NM. If it's a 12 DME arc, you would start your turn at 13.4 DME. This is a rule of thumb but works pretty well.
Once you're on the arc and at the proper distance (12 DME), start with the needle at the 90 degree postion. Continue until the needle is 5 degrees behind you. Turn 10 degrees in the direction of the arc so that the needle is now 5 degrees in front of you. Continue on that heading until it is 5 degrees behind you and repeat until you finish flying the arc.
If you have wind, you'll need to be thinking about what angle you'll need to crab for every 10 degrees of heading that you turn. This should be what you're doing anytime you're tracking an airway or direct to a beacon or whatever - always consider where the wind is, turn into the wind a few degrees (whatever you consider appropriate), and then monitor your track to make sure you applied the correct crab angle.