The first job is to make the baseboards fit together accurately, and for this I used Baseboard alignment dowels from C&L Finescale.
These comprise a male and female part, the male on one side and the female on the other, recessed into the baseboard end (25mm hole made with a forstner bit) and a clearance hole for the male part in the female end (8mm drill bit).
You can see the fitted dowels in the baseboard ends below, along with a couple more fittings to show what they look like.
To make these fit accurately I clamped the two baseboard ends together face to face, then drilled a 1mm marking hole through both of them at the marked points (50mm from the end and 30mm down from the top).
Then I separated the two parts, drilled the 25mm recesses (using the 1mm holes as a guide) and the 8mm clearance holes. It was then simply a case of screwing the dowels in using the supplied screws, but rotating the female part 90 deg so the screws did not clash.
Here are the two baseboard ends connected together by only the dowels.
As you can see the top of the baseboard ends is nicely lined up thanks to the dowels.
Also in shot are the 6mm pan head machine screws, penny washers and wing nuts that will be used to hold the baseboards together.
There is another way of fitting these dowels. The male part of the dowel has a conical point opposite the sprue and you can use this to mark the position of the female part. First fit the two male parts in one baseboard end, then reverse them so the point is protruding, clamp the other baseboard end to the first and the points will mark where the female parts should go, but I did not use this method.
Finally I cut a recess in the bottom of both baseboard ends for the electrical connectors that will carry all the wiring across the baseboard joints, see photo below.
These pluggable connectors allow 12 individual connections across the joint. I am using DCC to control the layout, but DC (16v AC) for points and accessories. However I am planning to have a mimic control panel on each baseboard so I don't expect to have more than 12 wires going from one baseboard to another.
So that is the first baseboard joint completed, only four more to go!