Step 1: Get & Hack Wood
Get to somebody in your family or to a friend or to a really friendly woodshop owner and have them cut the wood to the right lengths. To hard a job to do this with your regular DIY woodsaw ;)
See picture for lengths that I used, but feel free to alter where needed.
Step 2: Get Construction Materials
I found a workshop which makes these construction materials themselves from hardened steel. They had a nice black finish which would look good with the oak wood. A bit tricky to use because measurements needed to be perfect to within about 1 mm. Not on the picture but also used were three connector bolts (no screwing end, but two ends with a hole in it) to screw together the tops (see next step).
I'm sure anybody can find something similar to use and if you can't and don't really bother that much about any visual construction parts: just put a 4 x 4 cm length of oak wood left over from the cutting in the corners to screw together the legs, the tops and the supporting beams.
Step 3: Connect the Two Tops
To be honoust, this was pretty hard and it took me a while to get it right! In the end it was worth the time because this is the part you look at most of the times...
Step 4: Prepare Legs & Supporting Beams
Next step is to prepare the legs and the supporting beams. First the two outer legs, then the supporting beams and then the central leg.
This order seems illogical but actually is necessary to get accurate measurement. Since the sawing of the worktops has already occured there is no other option than to use the connection of the supporting beams with the central leg to get rid of any 'faulty' lengths of the working tops.
In my case it ended up beeing about two millimeters of missing length in the supporting beams. Hardly noticable when the whole thing is put together.
As you can see in the picture I used 'wooden connectors' (really don't have any clue as to what they are called in English!) to be able to position the legs and the supporting beams in the right place while taking measurements for the hardened steel bolts to construct & keep together the whole thing.
The hardened bolts are screwed into another bolt (the grey ones in the earlier picture) that is inserted into the bottom of the working tops. The hardened bolts than slot into a hole in the legs and into the nut that is already in the leg as you can see (that little black round thingie). Then you tighten the little centre piece in the nut and the leg and the top are tightend together really good. This desk might even outlive me; it's that tight!
Optional: I had to make it possible for the desk to be levelled. My floor is really old and therefor far from level. So I took some little feet from another Ikea closet, drilled holes in the bottom of the three legs (diameter 14 mm, 10 mm deep). Then I drilled a smaller, deeper hole (10 mm wide, 80 mm deep). In the wide part of the holes I put M8 nuts with a little bit of glue. Now you have very simple levelling feet to your desk. If you don't have those feet present, you can buy them for about 20-25 eurocents a piece at Ikea.
Don't underestimate the time it needs to do this. In total this took me about 1 1/2 days to get all the work done in this step alone. It takes a lot of measuring, drilling, fitting and sometimes correcting little mistakes to get it to fit properly.
Step 5: Putting It All Together
First you get all the stuff to the place where you want it to be. Then you connect the tops (part 1) and you connect the legs and the supporting beams to eachother (part 2).
The only thing left to do is to check the position of the holes in the hardened steel bolts (to see if they point in the right direction and if all the centre pieces in the nuts are screwed out so that the bolts can slot into them.
You need at least three people to lift the legs and supporting beams (part 2 that is) because of it's length (3 meters). When lifted walk over to the tops and gently lower part 2 over part 1. Usually this will take a little bit of fiddling to get all the bolts and wooden connectors to find there spot.
Then tighten all the bolts to make it into one desk!
Step 6: Finish Him!
Sit down, have another beer like the night before to congratulate yourself. Cheers!