I looked around online and found out that lots of people think cutting an opening in your poncho liner is a terrible idea, while others think it's dumb that they didn't come with an opening in the first place. Personally, I don't plan on going camping without a dedicated sleeping bag, but I can see the logic behind not cutting a hole in your liner in case you have to make a "Ranger roll" in an emergency. I found that some places make a conversion kit with a zipper, but as my past posts have shown, I'm too cheap for that

After a bit of searching, I found a place that sells ripstop nylon repair tape. This is heavy duty stuff - it's what those guys who fly those giant stunt-kites use for repairs. I ordered a few yards of it. So, to start the project, here's the ponchos & liners:

(I wanted OD, but they were sold out.

The other parts of the project are some outdoor rated nylon UV Resistant thread (should be available at any better camping/outdoor store), some velcro tabs (I got mine at the grocery store), and the aforementioned tape.

To start out, I laid out the poncho and liner, making sure they were lined up/correct sides facing each other, and tied them together at the corners.

Next, I located the head opening of the poncho, spread it open, and marked a short line with a ruler and a permanent marker:

At this point I removed the poncho, since it would just be in the way. I took a razor blade and a ruler and made a cut approx. 15 inches wide in the liner:

I measured the tape and cut a length equal to length of the cut:

...then I folded the tape over the cut:

I also added some tape athe ends of the cut to prevent fraying:

I will take an iron set on low and make a few passes over the tape to really set the adhesive.
To close the opening, I was going to use the velcro tabs. I didn't have faith in the adhesive backing on the velcro to stand up in the long-term, so I decided to sew it on. I peeled the adhesive backing off the velcro and peeled the adhesive backing off of a length of the nylon tape and stuck them them together:

...I then trimmed the excess tape off the edges with a razor blade.
I lined up the first tab where I wanted it (make sure you put the stiff side of the velcro facing up so it doesn't scratch you when you're wearing the poncho).

I threaded up the needle...

(the outdoor-rated rated thread is pretty stout, but not much thicker than regular thread. Good stuff) and stitched 'em on:

(Sorry for the crappy pic - my camera doesn't like to do close-ups). Basically I made a "box" with the stitches, then made an "X" through the center of the box. The poncho liner + velcro is kinda thick, so you might want to have a thimble handy. I used a combination of my thumbnails & my Leatherman to pull the needle through.

Here's how it came out:

Obviously it would still be potentially drafty as a blanket, but I didn't want to put velcro along the entire seam for reasons of comfort (and laziness when it comes to sewing). Of course, the black tape looks better on the black poncho liner, but the kite people don't make camo tape, and I photographed the camo one so you could see what I was doing in the pictures... anyways, I can't imagine I'll ever be wearing just the liner by itself, so the poncho will cover up the tape.
Edit to add: I finished the other poncho, and setting the adhesive tape with an iron came out GREAT - it's really, really on there good, and the ironing flattened it out and makes it look much cleaner.