I just cut mine with one of those really sharp snap-blade knives, the larger size one, extended all the way. Trimmed it to rough shape I wanted, which is easy enough to do if you don't get too aggressive or impatient, then used one of those little rasp pocket plane things to smooth it out. Makes some pink snow, which is why I'm so glad that Peter Sellers wasn't chasing a YELLOW panther. Don't need no yellow snow!
![Laughing :lol:](./images/smilies/econ_laugh.gif)
Just kinda lightly saw at it with this, and it'll slice off cleanly. It will dull, though, after about 50' of use. Still works well, just tends to snag a bit. Go slower
![Wink ;)](./images/smilies/econ_wink.gif)
![Image](https://s25.postimg.org/l64tfrsbj/20170715_114455.jpg)
Then I finished it off with the raspy plane thingys. A light touch is all that's needed. I've also found that holding it at a slightly oblique angle, not straight on, makes it cut much more controllably without skipping/chattering.
![Image](https://s25.postimg.org/msymeqtr3/20170715_120000.jpg)
The rounded corner, ready for the sander:
![Image](https://s25.postimg.org/j82qvip7j/20170715_120351.jpg)
(FWIW, after all that's done is when I fill the gaps, if any, with Alex Plus and hand-smooth it. Sander will booger up caulk.)