The MIs for our kitchen tap (which has three flexi tails, one extra for boiling water) say to put a little silicone grease on the O seals (the tap was supplied with a small sachet of the clear stuff) then fit the tails hand tight only up inside the base. The tails were long enough that I could feed all three down through the worktop hole from the top, one by one, so the ends didn't cause the things to jam in the hole (at a guess the flexi tails were around 18" long). Fitting the big plate underneath, plus the washer and long tubular nut to secure the tap was pretty easy after that.
We've had no problems with leaks, even though the tails are only hand tight. In fact you can unscrew the tails two or three turns and they stay sealed, as the O rings seal into machined bores in the base of the tap, away from the threaded part, so they aren't working under compression at all.