Soldering feeder wires to Märklin K-Tracks does not need to be difficult.
Solder the red wire to the small tounges under the track – be carefull and use as little solder as possible, so you dont solder the tounges together. Its also a good idea to solder in the small recess next to the black metal, so the wire fits nicely beneath the track.
Solder the brown wire to the rails, by soldering it to the railjoiners. Again – dont use to much solder, as it may cause a shortcircuit.

Sorry for the somewhat blurry pictures.