How easy is it to cut down a timber external door frame for use in a small door?
Wow, those are pretty spiffy! You get bridal joints at the header and mortise &amp; tenons at the threshold. Here in the US, we just get simple butt joints, often just stapled together.
If you're willing to put some elbow grease into it, making a new mortise for the tenons isn't too difficult. There are a ton of videos on the tube of you (Paul Sellers is a widely recognized, highly rated guy, also check out Mattias Wandell) on how to make a mortise &amp; tenon joint. Since you'll already have the tenon, you'll only need to remake a mortise.
The simplest way to do it with some hand tools is:

Very carefully mark out the location for the new mortise. Use the tenon from the side of the frame and mark on the threshold where it should go. This is, generally, more accurate than trying to measure.
Using a drill bit the same width as the mortise needs to be, drill a series of holes for the mortise, removing most of the material.
Using a chisel, remove the remaining material left in the mortise and square up the corners &amp; edges

You'll want to purchase just one chisel, preferably exactly the width of the smaller dimension of your mortise. If it's narrower than that dimension, it's fine, but wider will not work at all. This can be a cheap, off-brand chisel if you don't expect to do much more work like this in the future. There are chisels that are hundreds of &lt;currency units&gt; per chisel, you'll do just fine with one that's ten or less.
See also Paul's videos on how to sharpen - some cash spent on a single tile &amp; some sandpaper and some time spent sharpening (yes, even a brand new chisel is too dull to use easily) will more than pay for itself in reduced working time &amp; frustration.
Make sure that you do NOT oversize the mortise. You want it to be a snug fit. A loose tenon isn't your friend here. Though, since the jamb will be nailed/screwed into the door opening, it's probably not as critical as it would be if the M&amp;T joint were holding up your kitchen table.
You're better off starting out ever so slightly small then enlarging the mortise just enough to let the tenon slide in with some force. That's much easier than adding wood back in to a mortise that's already too big.


Follow the original assembly instructions to assemble your new, narrower door jamb.

Spend some time watching some videos to get the details and watch how it's done, this was intended to be an overview. I believe these instructions are complete enough to get you through the task, but, if you're not a woodworker, there are probably some gaping holes.
You probably don't want to use a door jamb set designed for interior use. The exterior jambs are either made of wood that is naturally rot resistant and/or are treated for rot resistance. The interior jambs won't be treated, and only the more expensive (i.e. &quot;custom grade&quot; or similar) ones will be made of an exterior appropriate wood.