To start with, remove the speedo cable inner.
Using the wheel end of the inner cable (the end without the brass collar), insert it up the hole at the underside of the forks...
As seen here...
Push the inner cable upwards until it appears out of the top of the fork tube as seen below.
Now push the wheel end of the inner cable (without the fastener), down over the inner cable, using the inner as a guide until it pops out of the hole at the bottom of the forks...
Now remove the inner cable from below and insert it again, the correct way round from the top.
Until the brass fitting is inside the speedo nut as shown below.
Feed the cable through the retaining clip attached to the forks.
Slip the front wheel on to the forks. It may help to prop up the front of the machine by using a block of wood under the stand feet.
Note: When the Lambretta is ON the stand, the front wheel should touch the floor.
Proper measurements made from the frame, stand and all the angles indicates it was designed to have both wheels grounded when the stand was deployed. If your scooter has the front wheel a little off the ground when on the stand, that is acceptable and it may even settle down with use. If there is a large gap of say an inch or more, something is wrong and the frame struts WILL be damaged if weight is put on the scooter, for example, by sitting on it with the stand deployed.
Push the rubber protective cover up the speedo cable.
Push the fastener up the cable and push the cover down on to it.
Insert the collar stop.
Fasten in to the speedo drive on the front wheel and tighten.
Now spin the front wheel by hand and the speedo inner cable should be turning in the headset if you look.
If it doesn't, ensure there is sufficient length of inner cable entering the front wheel and adjust the outer cable if required.