Pull the lace on each side of the snag gently to help pull the snagged threads back into position. A snag in lace can cause a gathered effect in the material or leave a loop of excess threads. Work the lace very carefully back and forth with both hands to make the tension even on the pulled threads.
Insert a small crochet hook through the lace from the reverse side. Use a very fine hook if the lace is tightly woven, to avoid creating a hole.
Catch the pulled threads with the crochet hook and pull them through to the reverse side of the lace curtain. This restores the smooth look of the lace in the front, without damaging the curtain.
Thread a needle with thread that matches the lace. Don't double the thread. Insert the needle from the back of the lace so the knot will be on the reverse side of the curtain. Hold the lace carefully, with the edges of the rip meeting.
Stitch the lace together with small, even stitches from one side of the rip to the other -- as though sewing up a wound -- taking care not to overlap the lace. By stitching the edges together as neatly as possible without overlapping the lace, the repair will show less.
Keep the thread tension even and finish sewing up the rip. Even tension helps to avoid puckers that would show in the lace. Keeping the stitches flat and not too tight helps the lace curtain to fall naturally. Take three to four stitches at the end of the rip, each on top of the other, to fasten the stitching securely. Stick the needle through the lace to the back side of the curtain and carefully cut the thread.