Measure and cut 3 x 3inch square of corrugated cardboard.Cut 3 pieces of same size of the cardboard to get required thickness.Now apply glue to the pieces of cardboard and join them.Take your Fabric and glue it on the cardboard pieces. i'm sewing it because my fabric is stretchy and i was not getting the required result by glue.and now your base is ready.
Step 2: Attaching the base with the main cushion
Now, for the main cushion,cut one more 3x3inch piece of cardboard.cut your fabric bigger then your cardboard piece.Glue 3 side leave the forth opening for stuffing the cotton or inserting the foam.after you've stuffed your cushion with cotton/foam,close the opening.and glue the cushion on the base you've made before.Glue the thread or ribbon at the joining line of the base and cushion. now its time to make the back rest of sofa.