Suppose you are using the Doppler technique to look for planets around another star. What must you do?

Compare many spectra of the star taken over a period of many months or years

Explanation:

Because of the of balanced center of gravity of the planets and the star wobbling of the star would be noticed. During this wobble the star will keep moving closer and farther from us. This change in distance will change the wavelength of the light coming from the star.

When the star is closer blue shift will occur and when it is farther from the Earth red shift will occur. So, the spectra must be compared over a long period of time.