Insert the parabolic leaf underneath your present single spring. Put on the new u-bolts and tighten. Ensure that the locator spud on the bottom of the main single leaf is going to fit into the locating hole on the extra leaf.
Also ensure that the locator spud on the bottom of the extra leaf will fit into the locating hole in the axle/seat. (If needed, the locating spud on the extra leaf can be reduced with a grinder if your locating hole happens to be too shallow).
We've been doing leaf springs since 1977! The item "Mercedes Sprinter 515 / LT35 / VW Crafter Leaf Spring Uprate Kit (Heavy Duty)" is in sale since Thursday, August 15, 2019. This item is in the category "Vehicle Parts & Accessories\Commercial Vehicle Parts\Vans/Pickups\Van/Pickup Parts". The seller is "leafsprings" and is located in Lisburn. This item can be shipped to United Kingdom.