yes you can do this yourself. i did. the brace installs from below. but you do need to install something to hange the fan from that will accept the weight of the fan. check
a standard home repair book or two at your library. and read the section cafefully. you do need to make sure that the brace is secure and the box is secure as well so the fan doesn't wobble.
diggerb

My understanding is that a brace is required if you are locating the fan in a place where there currently is not a box, and that location happens to hit somewhere where there is not a framing member to attach directly to the box. So the brace connects between the closest two wood rafters, and gives the electrical box something to support it.

If you have a fixture now in the location where the fan will go, you probably have a box already there securely attached to wood framing. You can probably see what's there when you take down the old fixture.

I've put many a ceiling fan in my old house where existing light fixtures and electrical boxes were, and no problem. Unless you've had water damage or issues that may have damaged the framing, you should be good. Once the fixture is out (and power off), you can check the stability of the existing box pretty easily.

"My understanding is that a brace is required if you are locating the fan in a place where there currently is not a box, and that location happens to hit somewhere where there is not a framing member to attach directly to the box."

This is not correct.
Most boxes are not rated to support a fan.
A box listed for directly supporting the fan is required.
The expanding braces come with a fan rated box, but other types of boxes for direct mounting to framing are available.
It will be marked as rated for fan support (often a sticker).