I had one to assemble a while back where the tails disappeared about 200mm up inside the base of the tap.
I found the easiest way was to dismantle the tap, fit the tails to the valve body and tighten them before re-fitting the valve body complete with tails in the tap.
i did this about a month ago to my new bathroom monobloc mixer! i bought flexi connectors 12mm to 15mm - the tap connectors screw inside the tap body (both hot and cold) and then i tightened with an adjustable spanner pretty tight but not over tights. then the other end connected onto the 15mm supply pipes. tigthened these pretty tight, but not over tight, using 2 adjustables!