Hello Behrad, the instruction for you will be different due to the output voltage needed is 50-0-50
Winding turns for you are primary 18 turns and secondary 6-0-6 turns. (volts per turn = 8.6v)
with these turns you will get around 52v peak on the output at around 230vac input.
Winding instruction for ETD49 N87 core
The primary and secondary should be made with 2 bundles of 4 wires each twisted together (8 wires total divided in 2 bundles) copper wire diameter is 0.5mm and frequency of operation is 65Khz
Put heat shrink tubing at the ends of the wire bundles when coming out with the wire from the bobbin.
1) Wind 9 turns of primary winding (2 bundles together)leave the rest out this should cover one layer leaving 3 mm margin at the ends of the bobbin. leave the rest of the wire out so that you wind the second part of the primary after the secondary.
2) Put 3 layers of mylar or capton tape to insulate.
3) Prepare 2 bundles of 4 wires each and wind half the secondary starting from the center tap and finish on the pins of the bobbin at one end.
Note put the winding in the center of the bobbin and fill the gap on the sides with a piece of tape cut to size to fill the gap between winding and the bobbin cheek so that you have enough to come level with the winding.
4) wind the second part of the secondary starting also from the center tap as in instruction 3. fill also the sides to get an even surface for the last part of the primary. Put 3 layers of tape to insulate.
5) Take the remaining wire left from the first half primary and wind the last half of the primary(9turns) and finish at the pins. insulate with 3 layers of tape.
Lastly you can wind the auxiliary windings as needed and calculate at 8.6v per turn ( example for 15-0-15 volts you need 3 turns bifilar (25v) then regulate with 7815 and 7915 for the preamplifier voltage. for 12 for fan motor supply you need 2 turns (17v) and regulate at 12v.
I suggest you take a look at this video so that you understand more of what am I saying in the instructions.
https://www.youtube.com/watch?v=_K3ixhcTYFg&t=5s
good luck. Silvio