Wash the fabric to get it to relax into the shape it will want to maintain. Then not worry about the grain.
Fold the fabric in half, selvage to selvage. Holding the fabric at the fold, pick it up, readjust the fold line until the selvages are together. Carefully lay the fabric on the cutting board with fold toward you.
Line up the ruller on the fold, not the cut edge. Use 6 or 8 inch wide ruller so there's enough room to be sure the fold is on the lines. Trim the fabric to straighten the cut edge perpendicular to the fold/selvages.