Hit your walk button and switch to slow walk.
Walk a straight line while clicking, it won't take long to learn how close to get them and the slow walk lets you plant them fairly close. You can plant them too close if you just button mash but you just need a consistent click-click-click to plant them perfectly.

Also noticed camera angle plays a big part in planting as well. I found looking down and close to the player, without being blocked by player, the smoothest experience.