I had five sets of blinds with this problem and set out to fix them rather than having to buy new ones. And I'm gonna show you how I did it. This may sound like a daunting task, but it is easy and worth it to repair shades that have nothing else wrong.
Step 1: Supplies and Tools
we searched several places looking for suitable cord. Lowe's and Home Depot didn't carry any and suggested the Blind specialty store which we avoided because they'd probably charge a bundle for it. We eventually went to Joann Fabrics and checked their upholstery/decor fabric section and found some polyester cord in the right size. It was white and didn't exactly match the color of our blinds but we don't care.
Use the ones from your old cords. If you lost them, you can use pretty beads or decorative knots or a combination of these. They add weight to the bottom of the cords so that they hang right and provide a grip for pulling.
They cut things.
For sealing the ends of the polyester cord
Used for pulling the cord in tight places. So much easier to use this then try to thread the cord.
Used for popping the end buttons off. You can use regular pliers, I just happened to have channel locks handy.
Step 2: Step 1: Prep
Set the shade on your work space and use the pliers and/or the nail file to pop the bottom buttons off. A twisting motion helps. These buttons hold the ladder tape in place and cover up the knot of the cord. They vary depending on your model of blind.
Pull the knot out and clip it off. Do this for all runs of cord. You can then pull all the old cord out.
Step 3: Step 2: Threading
Start at the run furthest from the little locking mechanism.
Thread the cord through the little hole in the bottom rail. I like to thread the cord through the slats in bunches, it saves time but it can be difficult if your slats aren't lined up correctly. I learned on my last cord run of my last blind that my paperclip hook could be used to line up the holes. I also like making sure that the ladder tape is on the inside of my cord. This is purely for aesthetics and it doesn't really matter to the operation of the blinds.
Thread all the way up.
Step 4: Step 3: Thread the Cord Into the Header Bar.
The goal here is to thread the cord through the opening and over the little metal bar. this bar keeps the cord positioned right and from wearing on the plastic. This is where the paper clip comes in. I tried several times just to thread the cord in on it's own, but the cord is floppy and doesn't like to cooperate, so I bent a paper clip into a hook.
Slide the hook into the hole, making sure it's above the metal bar and the hook is passed where you're gonna fish the loop you tied into the hole.
Stuff the loop into the hole (as pictured) and try to snare it with the hook. This can be the most frustrating step, but keep patient you'll get it eventually and it gets easier with practice.
Once you've got it hooked, pull it through and thread it through the other holes as well all the way to the locking mechanism.
Step 5: Step 4: the Locking Mechanism
Step 6: Step 5: Measure
Make sure you have enough to tie a large knot at the end and then cut the tail, sealing the ends.
Tie a regular square knot in the tail close to the end and stuff it and the excess ladder tape back in the hole and pop the button back in.
Pull the blind closed and repeat steps 2-4 on the other runs. This time to measure the cord length just measure it against the first one.
Step 7: Step 6: Finishing
If you lost your little cone thingys, you can tie a decorative knot, like the monkey's fist, or use some beads to finish it.
Put your blinds back up. Make sure your cords are long enough and tie a knot in them to keep them together.
Voila. You have repaired your shades.