OMG.

Doing the pleats themselves without tape is like making your own pie crust. Why....? You can buy the tape at Hancock fabrics or Joanne's. They usually have sewing notions like these where other stores do not.

1st and foremost: has your lady made pleat curtains before? There's a lot more involved than just dry running it with safety pins. Nice curtains will depend upon just how much effort put into them, and how much you tell yourself it'll be just "fine" or are willing to settle.

Yes, she (or you) can unhem one side of each and attach them. Make certain you're doing the correct right and left sides to be joined. The hem at the bottom doesn't need to be completely unhemmed, just opened to join the panels. I would completely open the rod pockets, though, because you're getting rid of them anyway. You don't need the bulk in your pleats and it gives you the opportunity to make the length you want.

Insist on pattern matching. Sometimes, if you don't say it out loud, it doesn't happen. When you buy the panels, lay them out on the floor and make certain they match up before you unhem then. I'm thinking plaids, stripes, or anything with a repeat. Like wall paper.

Take the width of your window and double it, and that's how much fabric you'll need to have them hang nicely when closed. When your drapes are open, if you have them off the window (or beside it) so the window is completely exposed, you must add those inches into the width of your window.

Decide how high your rods are going to be, taking into consideration the pleat hooks (wish you were close; I have 1000s of them) hang along a rod. Do you want them to graze the floor or puddle? You'll need those measurements. A professional seamstress and someone who makes curtains will insist on measuring themselves, just like a window, cabinet, door, granite, etc., installer.

Now, in my opinion, if you don't line these drapes they're not necessarily going to fall well. The pleats probably won't be very defined. Plus, lined, the weight is more luxurious and look like a far better quality. It also helps prevent dry rot on your decorative fabric. Dry rot happens on natural fabrics extremely quickly, synthetics not so fast. The sun is evil.

Remember. When you don't sew, build, wallpaper, paint, landscape, etc., the words "merely" and "just" are words that simply demonstrate that you probably don't have any idea what is actually involved.

Conversely, before you finalize your payments, as with any other contractor, have them installed and insist
that they hang properly,
that the patterns match or hang horizontally or vertically straight,
that they close properly and any pattern continues to line up,
that they're actually wide enough,
that you have matching ties (if you want them -- another panel)
that the grain is proper on a lining,
that the lining hangs smoothly behind the curtains, attached in the side seams, but free along the hem.
that you have curtain weights in the hem, that the top stitching is what you'd like it to be (thread color?)
that the lining and top fabric are comlimentary. Cotton shrinks, polyester doesn't. Prewashed?

I hope this helps a little. I'm sure you've already looked online at all the "Make your own drapes" articles.
There's a lot more that goes into nice curtains than one might think. Even in the articles, "just" isn't always that easy.

Lowe's has some pretty stuff! :)
Good luck.

If you want 144 inches of finished pinch-pleated draperies, then you need to start with 144x2.5 inches of straight fabric, for a total of 8 of your 50 inch wide panels. You can scrimp some, maybe down to 3 panels on each side, but I don't believe two panels per side will do it.

If you are paying a professional to do this for you, ask them how many you need.

I would avoid pleater tape like the plague. It looks messy, the pleats are never clean edged and the time you spend getting the pins into the pockets on the tape, you could have sewn the pleats your self....with a much neater and cleaner look. Before you buy the stuff....look at some draperies that have been done with tape and pleater pins....particularly lined ones.
I think double fullness in drapery fabric looks skimpy. 2 1/2 to 3 times the finished width looks a lot better when closed.
Your original question...you say you need 144 inches of finished drapery....that means to me that AFTER PLEATING the panel should measure 144 inches? is that what you meant? In which case you need 2 1/2 times 144 inches or 360 inches of fabric.

I own a drapery workroom and am asked to do this all the time, with the thinking that it will be cheaper to buy the ready made panels at say, Pottery Barn, and all I have to do is sort of join them together and make a pleated heading.

It's just not that simple.

If the finished width that you need is 144" wide you have to realize that each 50" wide ready made panel will pleat down to about 18-19" wide so you will need to buy 8 ready made panels to make the 144". If you're wanting typical French pinch pleats know that each pleat usually takes up about 6 to 7" of fabric. Sometime you can do a 5" pleat but it usually looks skimpy. Even if you want any of the decorative headings (not a pinch pleat) it takes up 1/2 to 3/4 of the width of the panel

In reality I have to spend a lot of time ironing to get the very stubborn wrinkles out from them being in the package. I charge for ironing. If you've ever spent seven, eight or nine hours ironing you would understand. I then remove the side hems, remove much of the bottom hem, re-align the grain of the fabric vertically and horizontally so it hangs nicely and most times I also need to remove the top heading. Somehow the heading that is there in the ready made panels never seems to work for pleating. Murphy's law always applies here.

If there is a fabric repeat that will take more time to join them together. If there is a lining involved that also adds to the complexity of things and the cost.

It looks real simple but as CEFreeman said, it isn't.

I would talk this over with your seamstress before buying the panels.

It is actually cheaper to buy fabric and have it fabricated into a finished window treatment of choice and it will look wonderful to boot.