Methods may vary widely depending on the profile, experience and equipment available, but here are some tips we have to offer.
All the best tools and experience will do you no good if you don't have your pieces cut to the right angle.
These handy devices work great for gluing a whole frame together. You let the glue set up and then nail it. They work on the same principle as a ratchet tie-down strap, making it quick and easy to change from one size to another. They are also very inexpensive, and as long as you have a well cut miter the frame will self-square itself. Then all you have to do is nail it.
While a v-nail works great in flat/wide profiles, we have yet to find a machine that will stack nails high enough to join one of our typical cap style mouldings. To hand drive nails into hardwood you will have to pre-drill. If you don't have a drill bit of the proper size (those little ones seem to have a way of breaking...), you can use one of the nails you're joining the frame with. Just grind or file the head off (parallel to the nail or it will end up too short), and put it in your drill. It may tend to burn a bit, and the going will be a bit slower but it is very functional and it's almost impossible to break a nail.
One other handy method of nailing is a power nail gun. You will want to look for either a Brad Nailer or a Headless Pinner. The nice thing about a headless pinner is that the hole it leaves is so small you won't even need to fill the hole in many cases. However you will HAVE to cross-nail (put one or more in each side of the corner), as they are not very resistant to pulling through.
Unless you are making finished corner frames from raw moulding it is exceedingly difficult to chop and join finished moulding and end up with flawless joints. Here are a couple tips to make them look better.
This is most important when using a moulding that has a finish significantly different in color than the base wood. Get an assortment of colored markers and color the edge of the miter cut with a color closest to the finish color. Then if there is a slight gap or misalignment it will be much less noticeable.
We will readily agree that no one likes to admit they putty their joints, but occasionally you may find yourself in a place with no other options. As long as the joint is substantially tight, and you have a mechanical fastener (nails or v-nails) you can safely fill in the odd gap. We recommend you get an assortment of colored waxes to do the job. They can be safely used on just about any kind of finish and any excess can be easily wiped off with a clean cloth.