cycle.travel isn’t really designed as a site for plotting routes you already know – it’s more for planning new ones, and suggesting the best routes between A and B. If you want a pure ‘plotter’ I’d usually suggest a site like RideWithGPS or plotaroute.com.

If you do want to use cycle.travel for this, though, you should be able to do it by making sure that ‘Click map to add more points’ is ticked and then clicking locations, but with a couple of provisos:

• The planner will always find its favoured route between the last place you clicked and the new one; if that’s not the way you want to go, you’ll need to drag that bit of the route to where you want it.
• If you click near the start point of the route (green marker), cycle.travel will generally assume you want to extend from the start of the route, not the end. This can be an issue if you’re plotting a circular route. If you do want to extend from the end, it’s probably better to drag your current end point instead.
• cycle.travel doesn’t like routing along busy roads or rough tracks – you can get round this to some extent by putting via points either side and setting the first one to ‘Go direct’, but again, other sites are probably better suited for routing along busy roads.