My bamboo fences won't connect, how do I make them work?

They have to be a certain space apart, take pole next to other and slowly drag out until square is all green with no red, do that till they connect

Place first fencepost by clicking on the ground wherever you want the post to go. Then place second fencepost, at a distance one fence-panel length away from the first post, by clicking on the ground again. Once you have the two fenceposts at exactly one fencing-panel length away from each other, then a fence-panel will appear between them; and this is where you should place the second post. Thus you now have one panel of fencing, and can keep adding posts until you have the amount of fence you want. (If the fencepost shows a red highlight around it you can't place it in that spot, you need to choose somewhere else)
If you are having trouble seeing where the second post should go to make a connecting panel with the first post: try holding the second fence post directly on top of the first post (when it will be showing red), then gradually move it away until it reaches a distance where a panel appears between the two (and a green square shows under the second post). That is where you can place it.
Please note: the game does not allow you to decide how far apart to place the posts. Each fence-panel is a set length. (Annoyingly, you cannot make shorter panels in awkward spots where you don't have much room). Also you cannot fence diagonally; the game only allows you to place fences left to right or up and down.
Also NOTE: fences cannot be placed where the panel crosses any obstacle (eg tuft of grass or tree). You will have to cut down anything eg grass, rock or tree that is in the way; or move it if it is something moveable like a building, animal or farm-plot). Usually the offending obstacle that is in the way will also be showing in red, but sometimes it is really hard to see what is in the way if you are trying to fence between some trees etc.
Hope this helps.