If you are not using a solar tracker, I guess you are going to fix the fresnel lens at a specific angle in both azimuth and elevation. The elevation angle should be determined by your latitude, although you may adjust this dependent on the season if it is for a shorter term test. The azimuth angle will only be normal to the sun at one time of the day. This will then introduce the minimum aberrations from the lens. If this is what you want, then the process for this is basically the same as setting a sundial, for which there are several instruction on-line.

I agree with George, but in practice it will be easier just to aim the lens for maximum power. Don't forget a fresnel lens will suffer from chromatic aberration, it might not itself be very flat, without additional supports and so on. If you are using a multijunction cell I strongly recommend you DON'T use a fresnel lens because the chromatic aberration will distort the spectral intensities of the sunlight which the cell is designed to expect. It might even seem that pointing the lens slightly off the sun will give more output - this is because such a direction gives a better balance between the colours, and closer to the balance that the cell is designed to work with.

Dear George and Tony thank you for giving your valuable answers.