A lot of people reckon that this is the most difficult task for a novice operator to learn and I tend to agree. The aisleway should be set up so that it's width is equal to the length of the truck and it's load plus a working clearance of 30 cm or 12 ins. (According to the RTITB the theoretical truck turning radius should be used but no-one does and neither do they at their training centre just down the road from me)!
The trainees should be taught the correct position in the aisleway for the truck widthways and when to start turning, which is usually when the truck's turning point is at the edge of the pallet as shown above. (Note that this turning point is different for three wheel trucks, reach trucks and combi trucks).
Trainees should practice this to the left and then the right and normally without a load on the truck to start with. They should be able to line up with the load on the floor on a fairly consistent basis.