Right, OK, I think this might do your head in though.
OPTION 1
(I assume that the tabs on the Ikea curtain are 4cm???) (I assume that on the vertical (as hung) sides of the ikea curtain the tabs are flush with the side of the curtain, there is no step)
Step 1:
Fold the sides of the long edge of the Ikea curtains inwards (i.e. towards the wrong side or indies of the curtain), and iron them so you have a marker line all down the long edges, the fold should be 4cm width (the same width as one of the tabs) all the way down. You have basically folded the tab in.
Step 2:
Cut blanket A to be 132cm wide. Leave the length until the end.
(same width as the ikea curtain (140cm) minus the width of the tab x 2 (8cm?) so final cut width measurement is 132cm)
Step 3:
Pin blanket A on the inside (or wrong side) of the ikea curtain so it's cut edges are level (or lie just within) the pressed creases, and it lies 3cm below the top edge of the curtain.
(Do you know how to pin so you can sew over the pins?)
Step 4:
Machine sew blanket A 1cm from it's cut edge along the side seams and the top edge, but don't sew the bottom.
Step 5:
Cut blanket B to be 128cm wide, don't cut the length until the end when you can trim it to fit more neatly.
(same width as the Ikea curtain (140cm) minus the width of the tabs x 2 for each side (16cm) plus 2 cm seam allowance for each side (4cm) (140 -16+4=128)
Step 6:
Lie the curtain down on the floor (with the folded pressed sides flat and open like it was before you ironed it) so the wrong side (or inside) is facing downwards towards the floor, and the right side is facing towards you, lay blanket B on top of the curtain so that it is flush with one edge, and 2cm below the top edge of the curtain. (the total width of the blanket will be 12cm narrower than the curtain, don't worry about this). Pin along the length edge you have lined up and sew along that edge 1cm from the edge. Do NOT sew the top edge
Step 7:
Lie the curtain on the floor exactly as it was before and pull the unstitched edge of Blanket B to line up with the unstitched edge of the Ikea curtain, and 2cm below the top edge.
Pin and stitch 1cm from the edge. Do NOT sew along the top edge
Step 8:
You will now have a tube of sorts. Turn the tube inside out, so that the right side of the curtain is on the outside again. Refold the edges you ironed earlier, and you will find that despite your doubts and confusion the blanket fits perfectly inside the new width of the curtain.
Make sure that the internal seams of the curtain lie flat and neat, then pin along the vertical seam lines, and stitch the length of the curtain on each side to hold the blanket lining in place.
Pin the top edge of the blanket all the way along and sew, it should lie 2cm below the finished edge of the curtain.
Step 9:
Cut the excess blanket fabric from the bottom edge of the finished curtain.
My head hurts