I inlet the barrel fully, then drill a pilot hole down through the channel where the entry thimble (into the fore end) will cover it up. This gives me a clear indication of the center of the barrel channel when I flip the stock over. I got used to doing this because on a couple of occasions I got into trouble measuring for the groove using the edges of the stock blank. With a centered pilot hole there is no doubt where the center is. Then, between that spot and the end of the barrel inlet I lay out the groove, and start with a round chisel, just like a barrel channel.
In about 5 minutes, I'm ready for the round plane which will make the groove nearly perfect. I make sure that the depth is correct. I always aim for a 1/8" web between barrel and ramrod, which is pretty standard.
The plane will only get so close to the plumb cut on the fore end - the rest has to be put in with a couple of round chisels. Above you can see the gap between the end of the groove and the plumb cut.
Once the groove is in and at full depth for the whole length (this can really mess you up if it either plunges or climbs at the entry point into the forestock), it's time to drill the hole. It gets interesting here, and scary for some people. I've certainly messed this up in all three directions - up into the barrel channel, down and out the bottom of the stock, and off into the lock mortise. They all mess up your day.
You can plug and redrill. You can put on a wear plate (probably why some of the original wear plates are there?) You can start drilling and correct a bad hole by using a smaller bit (say a 5/16") to point you in the direction you need to go. Or you can "aim small miss small", which is what works for me.
I have come to the conclusion that the hole will almost always go exactly where you aim it. It may go where you WANT it to go, but it will go where you AIM it. And I don't believe in the mumbo jumbo of special "non wandering" bits. This is just a 3/8" wood bit welded onto a piece of straight drill rod, and it does a great job.
Now, aiming the bit is not as simple as letting it ride in the groove, and I think this is a mistake a lot of people make. Sight down the stock - where is the bit pointing? Well, that's where it's going to go. It should come as no surprise. If I use a barrel with a significant amount of swamp in it, and maintain a 1/8" web the full length of the barrel, then I find it mandatory to bend the stock "up" in the middle - bowing it slightly - to in effect straighten out the bottom curve of the barrel inlet. Otherwise the front flare of the swamp will get in the way. If you keep the ramrod groove straight from muzzle to breech, to me the stock looks thick and unhandy.
Here is the stock upside down in the vices. It gets arched up slightly so that if you sight down the lines you've drawn on the stock, they're mostly straight. Sometimes I use the ramrod groove to guide the bit, sometimes I aim it by hand. But wherever you aim it, I can almost guarantee you that's where it's going to go.
I hand held the bit this time - eyeballing it to miss the wide mainspring on this lock and guiding it to hopefully leave a 1/8" web at the breech. Here is the result - it took 25 minutes from start to finish. I was low by 1/16". The top pencil line is the barrel channel - the next two are are where I was hoping to go, and the bottom darker black line shows the actual bottom of the ramrod hole.
It's a job that is completely manageable and I encourage you to tackle it. I always sort of get the feeling that the rest of the rifle is "downhill" once I get the hole drilled. Silly, when you consider how much else there is to do, but there it is.