Roz,
I apologize if you took any of what I said personally. I wasn't trying to disparage anyone's specific technique for cutting and mounting fillets. I was only making a feeble attempt to be honest with somebody who appeared to have no background nor any formal training in using fillets. I didn't mean to make an issue out of how anyone in particular mounts fillets.
Slick,
Here is, in detail, how I handle the fillet situation. Anyone who feels like it has my full permission to rip, shred, and otherwise tear apart my method with no malice from this end whatsoever.
1. First, as I said, it makes a cleaner presentation if you carefully mark on the face of the mat the exact dimensions of your opening lightly  with a penciland cut the opening with a reverse bevel. Make your cuts exactly from each corner to the opposite corner as any overcuts will show up in the finished mat.
2. If you have a fillet cutter (one of the best investments I made in my shop for small tools!), cut one end of the fillet. Lay that in the opening with the mitered end against the corner and mark the opposite end of the fillet for your second cut. Carefully chop that end and check for fit. Start with one of the long sides of the opening. That way, if you mess up and make the fillet too short, you can cut it down and use it on one of the short sides later.
3. Once you have one side cut, make the other long side and check for fit. The nice thing about a fillet cutter is you have a stop on most of them and can set the stop on the first fillet so that the second is cut exactly the same length. Then cut the 2 short sides. Lay all of them in the opening and check for proper fit. Shave off a schosh if needed to get a nice set of tight fitting corners.
4. I use the ASAP fillet tape that Dermot mentioned but most of the fillet tapes are good and have an extremely high tack to them. Tape the flat lip of the fillet (or the back of the mat if you are steady of hand and accurate of eye) and start installing each leg into the opening. I usually tape short "tracks" across the fillet and fill in the gaps with framer's glue or any good PVA glue. I use Maxim 5/15 and CornerWeld in my shop but there are many good brands of white framer's glue out there.
5. After you are done installing the fillet, cut some filler board (1/8" foam core works great) and build up the back side of the mat to the same lever as the fillet so the mat sits flat. Now,it depends on what you are doing with the mat package. If you are doing a double mat, attach the bottom mat to the built up top mat and you are finished. If you are doing a single mat with fillet, you should probably use a mylar tape or some type of barrier tape on the bottom of the fillet to avoid contact with the artwork. That fillet is wood and can acid burn an art surface in short time.
I have explained this off the top of my bald head so I may have missed something but that gives you a general rundown of how it's done. This is my way of installing fillets and, as I said before, there are other methods of doing the same job.
I would also re-emphasize that, for your own peace of mind and to further enhance your framing knowledge, you should really take some classes in those techniques that you feel you need some training. It doesn't hurt anything and you will find a wealth of information in most of the Decor classes or the PPFA classes at the various trade shows. I still take classes on a regular basis and I always learn something from them.
Knowledge is one of the few things that is completely yours when you have it! Nobody can take it away from you and you are <u>always</u> the better person for having it.
Good luck.
FGII