If you are reasonably handy at DIY there is no real problem. Tip the caravan to get maximum access to (usually) the front of the axle and make sure it is secure, brake on, wheels chocked and well supported in addition to the steadies.
Assemble the mover with all bolts only finger tight on a sheet of thick plastic in front of the caravan and then pull the whole thing under the van to the approximate position required.
Then lift one side - ideally with a trolley jack and attach to the chassis. when confident this side is secure, repeat for the other side. You will have to jiggle it about to get the correct roller to tyre clearance and sometimes this is easier by having the u bolts just gripping and tapping gently with a rubber mallet. Tighten each side in turn and check the gaps constantly. It is important that they are the same. Some mover makers supply a piece of wood to act as a gauge for this process - it should be sufficiently tight to hold itself in place but not too tight to remove by hand.
I should have said it may be easier to do the cable runs first, although this will depend on the van. I use flexible electrical conduit tube and pipe clips to keep the job neat - see www.vehiclewiringproducts.co.uk or similar suppliers.
Hope this helps. It's quite satisfying to fit your own and at least you know were all the joints and adjustments are for later servicing.