My Porcelanosa toilet seat cover loose. I want to tighten it. But I don't know how. Could you expert here can show me how? I want to attached the picture that you are able to see and show me but there is no option to attached. Could you please also advice. Thank you so much.

If the bolts spin and don't tighten, you'll need to use pliers to grip the nut on theunderside of the toilet while turning the top of the bolt with a screwdriver. Tightenuntil the seat stops wiggling. Fortunately, there's something you can do to keep thetoilet seat from coming loose so often
Loosen the screws with a screwdriver just enough so you can adjust the hinges to reposition the seat. Temporarily reinstall the seat and lid by pressing them back down onto the hinge pins. Close the seat and lid, and align them with the front and sides of the bowl.

Look where the seat attaches to the toilet bowl. There are 2 nuts underneath.
Those are the nuts you tighten. they will hold the seat firmly.

No do not use metal bolt and nut as this can crack the toilet. Just go to Lowes or Home Depot and buy 2 plastic bolts , 2 plastic wing nuts, and 2 plastic washers. Install with seat through toilet holes and tighten by hand until tight. I then use pliers to tight just a turn tighter and the seat should be center as you tightened one side, then the other, and back again until tight.

Usually there are plastic caps that snap down over the bolt head.
Shut toilet lid and get down close with a flashlight.

Seat attaches in two places.
Reach underneath and you can feel two bolts with nuts.
You don't need to see them, just feel with your hand.

The bolt head is covered with a plastic cover.
The cover is added for aesthetic purpose only.
Use small screwdriver or fingernail file or nail or scissors to pry up the cover and reveal bolt head.

Use bigger screwdriver to tighten bolts.
You may have to keep one hand on underside to hold nut while turning bolt clockwise.
Another trick is use hand to push bolt all the way up, and then finger tighten bolt most of the way, and use screwdriver for last few turns.

Tighten bolts good and tight, but don't overdo it.
Bolts will come loose again and you can tighten more later.

where the hinges sit, use a small screwdriver and prise off the top chrome cover.Under neath are the securing screws for the seat. tighten these carefully until seat is secure, then replace the covers.

If the screws don't tighten the either the screw threads are stripped or the attachment nut is stripped. Take the screw and nut to any hmoe repair center and get new ones. remember don't over-tighten then screws as this will crack the unit.

There should be a way to tighten the bolts without going that far Here's two things:
Look at the lid, with the seat down. The bolts are sometimes covered with a snap-in palstic cap or flap. If so, pry up with a screwdriver. Then use a screwdriver and wrench to tighten.
If the bolts appear to be cast as a part of the hinge assembly, you should be able to reach around the seat to the back side (no pun intended) of the toilet, and tighten with a wrench.
I sometimes use a 'mirror trick' to get a better idea of what's back there. Just use a small mirror and hold it to where you can see the bolt and not on the back.
Hope this gets you going (oops, another unintentional pun).
Best regards, --W/D-- please feel free to rate this solution..thanx!