By now you should have a good understanding of how 16th notes work within a beat. The next step in gaining control of 16ths is to use them along with rests. Here are some typical groupings of 16th notes and rests within a beat.
Once you feel confident with these rhythms, try combining them with other rhythms and with longer rests, as shown in the following examples. Count 1 e + a, 2 e + a ... etc with each example.
When you feel you have control of these rhythms on one note, try using them to create riffs, as shown in the next two examples. Memorise the rhythm on one note first.
Here is a riff using a two bar rhythm pattern. If you have trouble with the timing in this, or any other example, practice the rhythm on one note before adding in all the other notes.