At the point , the slope is . It doesn't depend on : it means that the slopes will be the same on any "column". The direction field is invariant by translation along the -axis. So you have to sketch one column of slopes (for instance at , or anywhere else), and copy it for a few other values of .
How to draw the directions? The main point is to see where the slope is positive or negative, or zero. Thus, as a start, you should study the sign of . Then draw the horizontal slopes (those equal to 0). You can draw the maximum/minimum slope if any, and add a few slopes to fill your sketch in and get a better feeling of what the field looks like.