I made mine at http://techshop.ws
Step 1: Collect some cardboard
You're going to need a cardboard sheet uninterrupted by cuts or flaps. The sheet should be at least the height of your trash can minus one inch, and its width should be the outer circumference of the trash can plus several inches. You'll be cutting the box into two pieces and gluing them back together, so you need use the extra width as overlapping surfaces to glue together. I had maybe 16 inches extra and thought that worked out pretty well.
The corrugations should run vertically top to bottom because that will help us bend the cardboard to the curves and/or corners of the sides of the trash can, and will give some much-needed strength to the pull tabs of the insert.
Step 2: Cut the cardboard box in half & rough fit it.
The reason we're making two cuts instead of just one and then coiling that sheet inside the trash can is this: most trash cans are slightly larger at the top than the bottom to help the trash slide out easily when the can is turned upside down. This means that if we wrap something around the outside (or in our case, the inside), the wrapping material will only touch the bottom at one point. If we cut the cardboard in half and make two half-wraps, that will give us two points of contact, one on each side of the trash can. This will help keep the insert stable and level in the trashcan.
Select either piece of cardboard and begin to mold it to fit into the trash can. You should be able to fold along the corrugations of the cardboard to bend curves if you have a round trash can, or sharp angles if you have a square trash can. I've found that using the dull end of a pen to score one line of corrugation makes it much easier to get a crisp bend. Don't push so hard that you're tearing the top sheet of cardboard, but its internal structure should collapse, making it flexible along that line, particularly if you're bending the cardboard towards the side that was scored.
Using this scoring technique as needed, fit the cardboard closely to the inside contour of one half of your trash can. If you're using enough cardboard, the ends of your fitted cardboard should extend past the center of the trash can on each side. We'll use this extra material as a surface to which to glue the second half of our insert.
Repeat this process for the second piece of cardboard.
Step 3: Sketch and cut some pull tabs
Measure the middle of the height of the insert and mark that as point A. Measure up three quarters of one inch and mark point B. At point B draw a line parallel to the top and bottom of the insert. This will be the fold line for the pull tab. Our goal is for the tab to stick up one inch over the top of the insert, which in a flat material like paper would mean we'd measure up one half inch, but when using a thicker material like cardboard you lose some distance in the fold, so a little extra material needs to be built into the calculations.
Now, measure from the top of the insert to the fold line. Measure the same distance from B down towards the bottom of the insert and mark a new point C. This point indicates where the tab and the top of the insert will touch once the tab gets folded up towards the top of the insert.
Measure three quarters of an inch down from point C and make another mark D. The space between C and D will be the height of the finger-hole of our tab. Measure another half inch down from D and mark point E. E will be the very top of our tab once it is folded up. If everything has gone approximately to plan, you should have just a little less than inch of space between E and the bottom of the insert.
OK. Draw a circle for the finger hole using points C and D as the very top and very bottom of the circle. Using that circle as a reference, draw the outline of your pull tab so that the tab extends from E all the way to the fold line we drew through point B.
When the say a picture is worth a thousand words, they weren't kidding.
Using an exacto knife, cut out the entire outline of the tab EXCEPT THE FOLD LINE. The fold line is the only thing keeping the tab attached to the insert. Make sure your cut goes all the way through the bottom of the cardboard, and be sure to protect the top of your table by putting a scrap piece of cardboard underneath the tab you're cutting out.
Once you've completed the cut, fold the tab along the fold line towards the top of the insert. The finger hole should be visible above the top of the insert. It is better to cut shallow and test your fold first since you don't want your tab to stick way up above the edge of the insert. Its easy to adjust the tab to make it longer. Shortening the tab is not as easy.
Once you've cut your first pull tab, repeat the process on the second piece of cardboard. To eliminate all the measuring for the second piece, just stack your two pieces of cardboard on top of each other and trace the tab cut with your exacto knife, transferring the line to the bottom piece which you can then cut.
Step 4: Glue the two halves of the insert together
Instructables user jack8559 had the good idea to apply glue to the overlapping areas as you would for either of the two original options. Put the liner pieces into the trash can, then put a trash bag full of water into the trash can, which will clamp the two halves of the insert together against the side of the trash can in exactly the proper position. Dump out the water once the glue has dried, and you're done! I haven't tested this method, but it sounds like exactly the way it should be done. Better make sure your bag doesn't have any holes in it though. Fill it up before you apply glue to the cardboard so it's ready to go before the glue is dry.
Option 1: Glue the pieces together while they're inside the trash can
Put the pieces into the trash can, apply glue to the surface of one flap of one half, and hold it against the overlapping flap of the other half. Make sure two halves are lined up so the tabs are symmetrical. Once the glue is dry, glue the other two flaps together. The advantage here is that you can see if you need to make adjustments to the two halves as you glue them. The disadvantage is that it's hard to get everything lined up properly, apply glue, then hold it down without having something slip out of place. Light weight clamps will make your life easier.
Option 2: Mark where the pieces overlap and glue them together outside the trash can
Put the pieces into the trash can and line them as they would be in their final positions. Use a pen to trace the outline of the overlapping areas of the two halves. Remove the pieces of the insert from the trash can and move to your tabletop. Line them up on your table top so they line up according to the lines you traced, and glue them. The advantage is that it's much easier to apply glue and hold it in place as it dries (just stick a book on it). The disadvantage is that you have to be accurate with your trace lines or your insert may not fit correctly after the glue dries.
Step 5: Personalize!
Now you can move on to tweaking your insert for your needs. Do you load your trashcan with heavy trash? You'll need to reinforce those pull tabs so the cardboard doesn't tear. Gluing a piece of cardboard along the fold line (between the tab and the insert) will help prevent tearing. Do you want the bag to be more securely attached to the insert? You can cut hooks into the sides of the hole left by making the pull tabs.
Is the whole insert too tall? Trim some cardboard off the bottom.
Do your tabs stick up too far above the edge of the insert? You can effectively lower the fold line of the pull tabs by gluing a strip of cardboard to the insert between the insert and the pull tab, very similar to the reinforcing mentioned above. The difference is that you would make a new fold line a little further down on the pull tab and cover over the original fold line with the cardboard strip.
Corrugated cardboard not sheik enough for you? You can use corrugated plastic for a more durable insert, or go pro and make one out of acrylic!
If you're frustrated by the tendency of the bag to balloon when you put it into the trashcan you can drill a hole in the back side of the trashcan to allow the air to escape. I prefer not to do this so that in a pinch I can still use my trashcans to hold liquids if need be.