There will be a point roughly midway along the boat (hopefully close to where the centreline is attached) that the boat pivots around when you steer left or right. To make a sharp 90 degree turn bring the boat to a stop, the pivot point should be either slightly behind or opposite where you are turning into but not ahead. Move the tiller 30 degs in the required direction, give the throttle a good amount of revs for about five to ten seconds then back into neutral and let the boat make the turn. The reason you don't stay in gear is to avoid the boat moving forward which would increase the turning circle. Disclaimer: This is what works for me, others will have different but equally valid methods.