I'm quite new myself, but what I usually do when learning any riff or lick, is that I break it down into as small portions as possible learn to play those, then when I can play them, I try playing them in succession. Usually, this means learning one measure at a time, unless it's a simple riff or lick like the ones in the first part of the Rock Course Level 1 lessons.

Generally, I've learnt that breaking a melodic phrase down into the smallest reasonable parts possible is the most effective way to learn riffs or licks made by others. It also helps me learn to play the phrase perfectly, as I concentrate my attention on that one phrase and rarely put down the guitar until I can successfully play it at full speed without messing up 5 times in a row, unless it's sufficiently difficult, at which point just settle for learning how it goes and then move on and return to it every time I pick up the guitar...

Of course, the best way for you to learn can be different from the best way for me to learn, so telling you what the best way to learn riffs and licks is may be difficult.