I measure the "back" of the bag and then subtract 1/2" from the measurement (to allow for the side pieces since I use 1/4" foam core). Then measure the 2 sides of the bag. Make sure you make the measurement a wee less because you
have to have space for the plexiglass front to slide in -- perhaps 1/8-1/4" less -- allows "wiggle room" for the bag to slide in and out. Measure also the height you want the insert to be. I take it up to just below where the cord handles come in.
Then lay out the foam core and measure out the height and mark. Then measure in from one end the width of one side of the bag -- mark -- measure the "back" measurement -- mark -- measure the other side and mark. Cut the foamcore out (DO NOT CUT THE SIDE MEASUREMENTS YET). Once the foamcore piece is cut out, then SCORE only the side measurements by cutting thru just one section of the cardboard and the foam -- leaving the other side of the cardboard intact. Now, you can fold the foamcore and you'll see the "box" for the insert. Carefully insert the "room" into the bag to see if size is correct. If you need to adjust the size, you can tape over the cut section and recut in the correct place.
I like to use this one-piece-scoring method for the back and sides rather than individual pieces. It is so much faster and easier to paper one large section and then fold, rather than wallpaper after the box is built (or paper the individual pieces and then glue the sides to the back because it is hard to glue them glued and look like a continuous piece).
Measure the section for the "floor" (the inside width measurement and depth measurement of the "box." I usually build up foam core sections by cutting 4-5 pieces and gluing them together to build up the floor to the height I want.
I usually do at least an 1." Test fit the box insert and the flooring into the bag once again, once the floor is made. However, DO NOT GLUE the "floor" in position yet. You may have to trim the edges a bit because you don't want
the box to fit too tightly in the bag.
Once the floor is finished and will fit snugly in your "box," you can glue in your flooring material (carpeting, flooring wood, or flooring paper) on the floor.
To paper the walls of the insert box, I use 3-M SPRAY ADHESIVE because glue or wallpaper paste will warp the foamcore. Lay out the side and back walls flat. Spray the adhesive on the wallpaper and apply wallpaper to the
foamcore. Smooth the wallpaper with your hands and then re-bend the walls to shape.
Glue the floor into the wall section. Put glue on sides and back of the floor unit and I usually use straight pins, or T-pins, to attach the around the bottom perimeter and sometimes a rubber band to keep in place till dry.
I use baseboard trim around the bottom to finish out the box. I also put trim on the top of the raw edge of the foamcore to hide the foamcore because I use a plexiglass top. If you don't want to look down into the room itself, you don't have to finish the top and just cut a piece of foamcore to top the box.
You can then crumble tissue, use ribbon, flowers, etc., to coordinate with your bag and glue to the top of the foamcore and place on top of your "box". Don't glue the top into place because you might have to get into the box
Now, you can cut the "front out of your gift bag." I usually cut 3/4" from the sides, up to an 1" or so below the cords are on the gift bag (below the doubled over section of the gift bag); and 1-1/2 up from the bottom of the gift
bag. Mark -- use with a sharp Exacto knife being careful NOT to cut past the intersecting marks. Always INSERT A PIECE OF CARDBOARD THE SIZE YOU NEED INTO THE BAG SO YOU DON'T CUT THE INTO THE SIDES OR BACK WHEN YOU CUT OUT THE FRONT PIECE.
If the bag has an oval or decorative edge you can cut around that – the opening doesn't have to be a rectangular area. If you'll look at some of the bags I've made (http://fayzerplace.homestead.com/GiftBags.html or http://fayzerplace.homestead.com/BabyGiftBags.html), you'll see where I left some of the "stars" on the baby gift bags, and the animals on the vet bag (http://fayzerplace.homestead.com/vetbag.html).
Place your furniture and other decorative accessories in the insert box and when finished carefully side the box into the bag -- being careful not to rip the corners where the opening is cut out.
Cut the plexiglass to fit the front opening and the top opening, if you are not using the foamcore top of the bag.