How do you construct a jk flip-flop from a t flip-flop?

Its very simple. All you have to do is short J and K inputs i.e. same inputs go into both J and K.
Actually T flip flop is the special case of J-K flip flop when both the inputs of J-K flip flop are equal to 1 ( or logic high) j=k=1. Know electronics has well explained about its designing. A very good use of this flip flop or J=K=1 condition is in counters.

The J-K flip flop is universal and can be used to make any of the other times. I agree with know electronics and VKP that it can be used in counters. Adding the practical use of conversion here . When you are working on ASIC or SOC Design, In the ECO stage if you want T or D flipflop and you have only JK flipflops then this conversion techniques can be helpful. similarly MUX conversions are helpful in realizing computational gates