If you don't want the bother of lining them, you can buy ready made linings that you hang separately. One advantage is that you can use them with other curtains if you want a change. Several places sell them, Amazon, curtain shops, markets. Here's one stockist..
in this case bagged out just means seams on both sides of the curtains , with seam edges facing the inside - if that makes sense - sort of like a cushion, but open at both ends
& blind hem, just means a hem where you don't see the stitching, with curtains personally I would normally just double fould the bottom edge to the length you want, press & an over stitch along the hem edge by hand
Personally I would ignore the fact they are already hemmed & make a proper job of it, IYSWIM - it's going to look far neater & stand up better to wash/cleaning.
1.Carefully the hems at the side & making sure your lining is aprox 4cm narrower than your now unhemmed curtain,
2.bag out the lining with the curtain fabric & press so that you have a 2cm(ish) bagged out blind hem at each side of the curtain
3. Fold the top edge over to the lining side, making a fold of about 4cm depending on the finish you want & press
4. Sew your curtain tape about 3cm in from the top pressed edge & again along the bottom of the curtain tape - making sure your tape is a few cms longer than the curtain edge, so that you can also fold over the cut edge & stitch as you go to neaten it too.
5. Press & hang for a while, - this allows cloth to find its level - now Pin along the curtain at the length you want them to be & hem
Hi, hoping for a bit of help. I have some ikea curtains which I was just going to hem and add heading tape to for my daughters room. I now realise I will need some kind of lining too. What is the easiest way to add this lining considering the 2 sides of the curtains are already hemmed, I just need to do top and bottom IYSWIM. It doesn't need to be blackout lining, but something reasonably substantial to help block a bit of light. Thanks for any guidance!