OK - this post is really hard to write. Explaining on paper when I need you here so I can show you is a pain......I will do my best.
I find shaped bits of trim a real challenge. Don't be put off by that - the level of difficulty will be determined by your own degree of spatial awareness. I have none! I have no sense of direction - often get lost just exiting a bathroom in a restaurant! No kidding.
I find it impossible to visualise something in three dimensions and always need to 'build' a mock-up to understand the issues. For example, I thought I wanted to change the rooms and interior walls in my first dolls house so I drew floor plans but then I had no way of being able to figure out how a dog-leg staircase would fit into the plan, or even how they 'worked'. I literally built a giant cardboard mock up of the whole house and staircase. I was horrified to discover the sorts of issues my 'design' had created in terms of windows and doors and access and lighting, none of which I was able to visualise on a plan layout.
So, if any of that sounds like you, then you will find shaped trims a bit challenging - not actually difficult, but needing to be thought about.
Here's how I cut a strip of narrow coving which was going to the back ceiling/wall of a room.
Firstly you need to measure the precise width you will need. I have tried all kinds of ways of doing this. You can generally get a six inch ruler in the space and make marks as you go and then tot up the measurements to get the width. You can do reasonably OK with a fabric measuring tape but that is not so precise as the rigid ruler.
What you must not do is assume that the nice easy to measure front of the room space will do because the front must be the same as the back. Even the floor at the back, which is also easier to measure than the ceiling line, will not do as there is no guarantee that it precisely matches the ceiling above.
The measurement needs to be along the actual ceiling edge and from wall to wall.
Here's what I use:
|strip of magazine paper|
I pull out the centre page of a magazine and stretch it across the space, one edge against one wall and make a crease where it meets the other. I now have a 'template' for the strip. No measuring with fiddly rules and no messing around transferring the odd 1/32"th of an inch or whatever measurement you end up with.
I then physically take the piece of wood to the room and look at the space and write on it which part of it will glue to the ceiling and which part of it will glue to the wall. If you have a fancy(ish) piece of coving this will matter as the top and back sections won't be the same width or routing. I then imagine what the cut on each of those 'planes' need to be like.
The wall one needs to have a straight vertical edge to run down the wall in the corner. I draw a line to remind me of that. These are not lines you will cut on at this stage just a reminder of how you want the strip of wood to end up after you've cut it.
|imagine the line drawn rather than already cut|
The ceiling face needs to have a forty-five degree cut in it which has to face inwards ready for another piece to join up to it. I draw a line again to remind me. Apologies again for showing you a finished cut rather than the pencil line .
|45 degrees facing inwards|
I then go to the mitre box and match the cut up to the cutting guides and cut the wood.
You can probably just see the line I drew to remind me of the direction of the angle of the cut. The wood is in the box with the wall plane to the back of the wall of the box and the ceiling plane looking at me on the top.
|can be done better|
This is where you can do better. My husband, who was taking the pictures for me, said "Why are you holding it like that, balanced on the thin edge? Just flip it and put the wood down with support underneath and at its back". He can 'see' this stuff, I can't. He was right it can be flipped and cut in a better way but, sadly, I will probably continue like this because it works for me.
|put your wall edge - the straight part - on the end of your template|
|go down to the other end and mark up with a pencil where you will make the other cut|
When they are glued in place this is how they will look on the ceiling:
|ignore the tacky wax oozing out of the join - this is just a demo piece|
After I had typed this I roamed the web to see if someone had done some simple and definitive instructions - these seem like a good set to have: Cutting cornice
Don't worry about it - I have made it sound worse than it is - do have a go.
These are internal cuts as they are on the inside of the walls. If you are wrapping a trim round a chimney breast for example they become external cuts.
If you use the diagrams I have given you a link for, it should be hunky dory. Otherwise, use my primitive and bordering on pathetic method of: taking stuff to the object, having a bit of a think about how it should end up and making marks to show that, before trotting off to the saw and mitre block.