Soil conditions can vary from one location to another, so you will  likely have to experiment to see what works best.  But there are some  general facts regarding soil preparation that are important to know. 

If you are preparing a field that has been in pasture  or uncultivated for a number of years, you will have accumulated a  large seed bank....that is a large number of weed seeds that have not  germinated.  Although a filed may look good after it is rototilled, the  weeds will appear in large numbers once the conditions are right for  germination.

The best approach is to prepare the field a year in  advance.  Rototill the field and let the weeds grow, and plow or  rototill before the weeds go to seed.  Doing this several times will  significantly reduce the amount of weed growth that you will have.

The  picture below shows a portion of a field that was heavy with weeds, and  the number of weed seeds that germinated once the conditions were  right.  A month earlier this field looked like good plantable black  dirt.