Get to know the following rhythm, write it out on paper:
Dotted eighth, sixteenth, eighth, eighth, sixteenth, dotted eighth.
Tap/clap with your hands.
Now practice the following tapping pattern with both hands (where T means tap both hands at the same time, R = right hand, L = left hand)
T R L R L R
also the inverse
T L R L R L
Now put the two together, tap the pattern using the rhythm.
Next to add verbal counting.
Count 1,2,3,4 - while tapping the hand pattern in rhythm, it will align with one of your hands.
Count 1,2,3 - while tapping the hand pattern in rhythm, it will align with the other hand.
Go through all variations of the above, flipping between R/L hand patterns and counting patterns. Be able to flip to another variant without stopping and restarting.
Want more?
Do all of the above while walking. It will give you a 2-3-4 polyrhythm. Of course, you need to do the variants as well without stopping.