Change one thing
A comparison answers a question only if one variable differs. Decide what yours is before you start. Two oolongs from one maker isolates the leaf. One tea brewed at two temperatures isolates temperature. A spring and an autumn pick of the same tea isolates season. Two unrelated teas are pleasant to drink and teach you very little.
Make everything else identical
Weigh the leaf, rather than estimating it. Use the same water, the same vessels, the same volume and the same timer. Start and stop both brews together, and pour them off completely, so that neither keeps steeping while you taste the other. Label the cups underneath.
Taste in a fixed order
Look at the dry leaf and smell it. Smell the wet leaf straight after pouring. Look at the liquor. Then taste each cup while it is still warm, and again as it cools, since the character moves with temperature. Rinse with plain water between cups, and go back and forth rather than finishing one first.
Keep notes that sort the senses
Give each sense its own line: what you smelled, what you tasted on the tongue, how it felt in the mouth, and what stayed after swallowing. Write the difference between the cups as well as the description of each. "B is drier and shorter than A" will be useful a year from now in a way that "nice, floral" will not.
Make it harder
Once this is easy, stop knowing which cup is which. Have someone else pour, or shuffle marked cups, and name the difference before you look. If you want a sterner test, use three cups where two are the same tea, and try to find the odd one out. Sensory scientists call this a triangle test, and it shows whether you can detect a difference at all, before you try to describe it. Repeat it, because one correct guess in three is what chance gives you.