Short version: decide roughly where you want to go and how long you've got, pick or draw a route that fits, check what the ground is actually like along the way, and save it to your phone so you can follow it offline. Do those four things and you've planned a route. Everything below is just how to do each one properly.
Planning a route sounds like it should need a wall of gear and some sort of qualification. It doesn't. Whether you're heading out for a gentle hour on foot or a proper trail run, the process is the same, and most of it is common sense with a map. Here's the whole thing, start to finish, in the order you'd actually do it.
Where do you actually want to go?
Start by picking the shape of your route, because that quietly decides most of the other choices.
There are three you'll meet again and again. An out and back, where you head out, turn around at some point, and retrace your steps. A loop, where you come back to the start by a different way. And point to point, where you finish somewhere else entirely and need to have thought about getting home.
When you're starting out, a loop or an out and back will serve you best. An out and back is the most forgiving of all, because you can turn round whenever you've had enough and you always know the way back. Loops are more interesting once you've got the hang of it, and worth a piece of their own if that's your thing (see how to plan a circular walk).
How far or how long should it be?
Think in time, not distance, and take the hills seriously.
Distance is a bit of a liar out here. A flat mile along a river and a mile up and over open moorland are not remotely the same effort, and the thing that catches people out is elevation. When you're weighing up a route, pay as much attention to how much climbing it involves as to the number of miles. Pick something you're confident you'll finish comfortably, then stretch it once you know how your legs feel about going uphill.
Runners, a word: trail pace has almost nothing to do with road pace. Rough ground, gates, mud and climbs all slow you down, so don't plan your first trail route around your usual 10k time. You'll have a much better day if you don't.
How do I plan the actual route?
Either draw your own line on a map, or start from a route someone's already done.
A route-planning app lets you do both. You can drop points and draw a line exactly where you want to go, which gives you full control. Or you can browse routes other people have already recorded and walked, which saves time and, more usefully, reassures you the path actually exists. That second point matters more than you'd expect, which brings us neatly to the next bit.
Is the path actually there?
Before you rely on a route, check the surface, the climb, and whether it sticks to real paths.
Maps are optimistic. A right of way on the map can be a nettle-choked rumour on the ground, and a tempting shortcut can turn out to be a stream with ideas above its station. Have a look at the elevation profile so there are no surprise walls of hill, check the route follows established tracks rather than striking off across open nothing, and if the app shows other people's recordings running along the same line, take that as a good sign it goes through.
Save it so you can follow it offline
Download your route and the map around it before you set off, while you've still got signal.
Signal has a cruel habit of vanishing exactly where the good walking and running is. An online map that needs data will happily freeze at the one junction where you needed it. Save everything offline before you leave and your location shows up on the map wherever you end up, no bars required. It's worth doing even on routes you think you know by heart. There's more on why in do you actually need offline maps for hiking?, and what to do when it does happen in what to do when you lose signal on a trail.
The stuff to sort on the day
Check the weather, tell someone your plan, start with a charged phone, and carry a little more than you think you need.
None of this is dramatic. A couple of thin layers you can add or shed, water, something to eat, a waterproof if there's any doubt in the forecast, and a power bank for longer days. Let someone know where you're going and roughly when you'll be back. It's the difference between a good day out and a faff, and it takes about two minutes. If you're brand new to this, there's a gentler walk-through in planning your first proper countryside walk.
Look back at where you went
Reviewing a route afterwards is how you get better at planning the next one.
Once you're home, seeing your actual distance, your time, and where the climbs really were tells you what you can comfortably handle. That makes your next route better judged, and the one after that better still. It's also, quietly, one of the nicer parts. There's more on making sense of it in what your route data can tell you after a walk or run.
Start smaller than you think
If there's one planning mistake almost everyone makes, it isn't getting lost. It's biting off more than the day can chew, finishing exhausted, and putting off the next one for a month. Plan something you'll finish still wanting to go again. Ambition can wait until route number four.
That's the whole job. LaceLock is being built to make each step of it simple. Draw your route or start from one you like, save it and the map for offline, follow your position live on the day, and have a proper look back at where you went once you're home.