How to Properly Learn a Difficult Riff: The Small-Fragment Method
Learning a new, difficult riff whole, over and over from the very start, is the most intuitive way, but not the most effective one. The small-fragment method works faster almost every time, no matter what specifically makes the riff hard (see [[practice-fast-riffs-with-metronome:the article on playing riffs with a metronome]] - that one is about the speed of an already-learned riff, this one is about the learning process itself).
Why starting from the beginning every time is inefficient
Replaying the whole riff from the first note every time means the easy opening gets drilled dozens of times, while the hard fragment in the middle or at the end gets exactly as many repetitions as everything else, even though it needs far more. This doesn't double the time spent on the hard part - it gets spent on the already-solid opening instead.
Break the riff into 2-4 note fragments
Split the riff into short fragments - natural boundaries are usually where the chord changes, a new bar starts, or the technique shifts noticeably. Each fragment should be short enough to hold in memory as a whole, without checking the tab on every single note.
Take each fragment to automatic on its own
Drill a fragment in isolation until it sounds clean and even with no conscious effort - only then move to the next one. This is slower at the start than immediately trying to play the whole riff, but overall needs fewer total repetitions of the genuinely hard parts.
Connect fragments two at a time, not all at once
Once two neighboring fragments are each solid on their own, connect exactly that pair and drill the transition between them - the same principle as connecting pentatonic positions (see the article on connecting positions). The transition between two already-familiar fragments almost always comes easier than expected, as long as both are genuinely automatic.
Assemble the whole riff only at the end
Only play the complete riff once every pair of fragments has been connected - and for the first several times, do it at a reduced tempo even if the individual fragments already sound fast on their own. Assembling the whole is a separate skill, not an automatic result of every piece being ready on its own.
Try it in practice
Open the metronome