We should remember that the boards are for the tabernacle; they are not the tabernacle itself.  It is the innermost curtain that is the tabernacle.  This is clear from Numbers 3:25 and 36.  The Gershonites carried the tabernacle, but the Merarites carried the boards.  So the boards of the tabernacle are the boards belonging to, and serving the interests of, the innermost curtain, which was the dwelling-place of God.  Over this tabernacle were the goats’ hair curtains, which made the tent or covering of the tabernacle., Exodus 26:7.  Then there was a covering for the tent consisting of rams’ skins dyed red, Exodus 26:14.  Finally, the “covering above”, Exodus 26:14, of badgers’ skins.  (Notice in passing the position of the apostrophe in the words goats’, rams’, and badgers’, telling us many animals were involved.  If it had been one animal in each case it would have been “goat’s”, ram’s, badger’s).

 The boards served the same purpose as tent poles, to keep up the fabric of the tent, but because they were substantial boards, standing in a line, they also gave structure and shape to the tabernacle.

 As the boards were “standing up”, we know they were vertical in use, even though their height is described as their length.

All 48 boards are of the same size, as are the sockets on which they stood, so the corner boards were not T-shaped or L-shaped as some suggest.

The following is a suggestion about the way the corners were arranged:

Exodus 26:18  And thou shalt make the boards for the tabernacle, twenty boards on the south side southward.

This is the first side, (because the other side is called the second side in verse 20), so Moses is writing as if he is coming through the gate on the east, and the first side is on his left.

26:20  And for the second side of the tabernacle on the north side there shall be twenty boards:

So we now know what Moses means when he writes about the sides.

26:22  And for the sides of the tabernacle westward thou shalt make six boards.

Now comes the difficult part!  We may speak of the end of the tabernacle, by which we mean the 6+2 boards on the west, but the Scripture does not speak like this.  I suggest that the expression “sides of the tabernacle westward” means “the sides of the tabernacle, (that is, the first and second sides of verses 18 and 20), at their westward ends”.
The six boards are said to be “for” the sides of the tabernacle westward, so they serve the interests of the first and second sides by giving them stability, and also by keeping them at their correct distance apart.  Remember there is nothing like this at the east end to stabilise the structure, so everything depends on the west end being stable.

26:23  And two boards shalt thou make for the corners of the tabernacle in the two sides.

We learn at least two things here.  First, that the two boards are for the corners.  Second, that they are in the two sides.  This would refer to the two sides, the first and the second, that form the south and north walls of the board structure.  So the two boards are in the sides, not in the ends.

26:24  And they shall be coupled together beneath, and they shall be coupled together above the head of it unto one ring: thus shall it be for them both; they shall be for the two corners.

We may understand by this the following:  “The head of it” refers to the westward-facing ends of the line of boards, which, because they are now forming in Moses’ mind as he writes, can be described as “it”, for the structure is becoming a reality.
Both of these particular boards are placed in the same way, for “thus shall it be for them both”.  They each, then, are coupled together to the head or end of their respective side.  And this coupling is done “beneath”, and “above”, and “unto one ring”.  By “one ring” I understand “ring number one”.  In other words, the first ring, in the first board in the line, through which the bars were inserted.  So the corner board is coupled together with the board adjacent to it by the ring that was inserted in the boards to hold the bars.  Then the first and lowest ring is lashed (presumably by fine linen cords, “all that serveth thereto”, Numbers 3:36), to the first and lowest ring on the six-board section that we often call the end.   In this way the three boards forming the corner are doubly fastened, firstly by the rings that pair them together, and then by the ropes that keep the corners square and rigid.  This is done beneath, and above, in others words, the fastening is done by means of the lowest ring and the topmost ring of the boards.