Suppose you have opened your rear left door,you are standing outside the car, now insert your right hand inside the gap between the seat(on which you sit) and the backrest(on which you lean your back) and hold the seat tight with your grip.
Now get your left hand below the left corner of the rear seat,just above the corner plastic mould on the small platform on which the rear seat is resting.You can insert only half of your fingers through that small gap.Now tightly grip the seat from below also.
Now pull the seat outwards and upwards with your both hands firmly,with a strong jerk.The corner spring-type pin of the seat will come out from the hole inside the platform below the seat on which it is resting.
Now repeat the same procedure on the rightside also, and take out the whole seat out.Now you can see the seatbelt buckles below it.There used to be holes on the seat at the end which matches with the backrest. Hope it is blocked due to your new seatcover installation(its a common mistake done by most seatcover installers).Now you have to cut the seat cover at the points where holes are present.But you have to again get the seatcover stitched around the corners of the holes ,cut by you so that the seatcover is firm on the seat.
Now insert the buckles through the holes.
Now you have to fix the seat to its previous position,firmly.For that first you have to insert the end ,through which you have inserted the buckles, below the backrest.Now you make the seat rest on the platform freely so that the corner spring-type pins are placed just above the holes on the platform,from where you have pulled them out.Now press the spring-type pin so that it bend little and enter the hole,repeat the same one-by-one on both sides.
Your job is done with the seat firmly in position and buckles visible over the seat at the corner between the seat and the backrest.
Do the same and tell me if you face any problem.