First, decide if you are going to use 1/8 or 1/16 notes. Or you can work with both.
The second step is to decide the form of your riff. Below are the main forms your riff can take (of course there can be more). We will concentrate on the first 3 notes. You can use the notes of the given chord to build the riffs, but is not 100% mandatory to stick to them, you can also use nearby notes.
The V method (1/16 notes)
The V method - upside down (1/16 notes)
For both these methods continue with the form until you make one measure, but you can variate the ending. Remember that the distance between the 2 consecutive notes can be as long as one octave.
The broken chord method (1/16 notes)
For this method copy the form 2 more times until you make one measure (variate the next 2 forms).
The chord method (1/8 notes)
For this method copy the form more until you make one measure but change the intervals or use chord inversions.
The linear method (1/8 notes)
For this method add 3 or 4 more notes (1/8) until you make one measure, but change the pattern (don't keep it linear).
The pattern method
Start building a pattern (let's say for the first 4 bars) and then you're copying the pattern over the next 12 bars respecting the harmony rules (chord progressions). Or you could create a double pattern (first 8 bars).
The no rules method
Build a catchy riff on your piano keyboard. Don't stick to any rules!
