In the picture to the left a SFH 610A-4 is shown from above. A groove or a dot marks the pin with the number 1. From here counting proceeds counter clockwise. 

Straighten the leads of the opto-couplers...

...and glue them together bottom to bottom with some instant glue in a way that the printing on top is still visible. On one side the pins 1 and 2 from one opto-coupler must face the pins 2 and 1 respectivelly of the other opto-coupler. On the other side the pins 3 and 4 must face the opposing pins 4 and 3.

Bend the lead pairs of pins 1 and 2 together.

Maybe you have to shorten the leads from pins 1 and 2 to avoid short-circuiting the connectors in the 3.5 audio jack. 

Unscrew the cover of the audio jack...

...and remove the ground connector...

...by bending it forth and back several times.

Solder the opto-couplers to the two remaining connectors of the audion jack in a way that one pin 1 and 2 pair each is soldered to one connector. Use an extend of solder tin to increase mechanical stability as the final cable has no cable relief.

In the picture to the left a ready-made cable with a 2.5mm audio jack on one side is shown. Many types of cameras use such a jack for connecting a shutter release cable to the camera. If your camera needs a different connector or a different pinout please use an appropriate cable instead.

Now solder the ground wire of the cable to the two pins 4. The shutter wire (tip of the 2.5 mm audio jack) has to be connected to pin 3 of the opto-coupler whose pin 1 is connected to the tip of the 3.5 mm audio jack. The focus wire (center ring of the 2.5 mm audio jack) has to be connected to pin 3 of the opto-coupler whose pin 1 is connected to the center ring of the 3.5 mm audio jack. If this sounds somewhat confusing, there's a 50-50-chance for you to connect the two wires in the correct orientation. So next thing to do is to check the proper function: start DSLR Remote and plug the 3.5 mm jack to the audio connector of your smartphone/tablet. Plug the 2.5 mm jack to the matching connector of your camera and switch on the camera. If not already done choose the remote type Cable in the settings of DSLR Remote and move to the remote frame. You should find two buttons here: S for Shutter and F for Focus. If you press the button F now your camera should focus. If it does you did connect the wires in the correct way. Now check the shutter function by pressing the button S to make shure this works as well.

If your camera takes a shot when F is pressed and just focus respectivelly if S is pressed you switched the wires und have to solder them the other way around.

To achieve a higher mechanical stability you should cover the whole with glue or hot glue. By doing so you get some sort of cable relief, to protect the soldering joints from beeing torn off by drag on the cable during usage. Finally for a better look you can cover the whole with duct tape or heat-shrink tubing.

Here the cover of an abundant bigger audio jack was placed over the opto-couplers and the upper part of the 3.5 mm audio jack. Finally it was filled with epoxy resin.