Loch is pronounced with a khhh sound made in the back of your mouth.
Find the part of your mouth that says 'k' and say 'k' a few times. Then find the part of your mouth that says 'h' and say 'h' a few times (they're the same part of your mouth, btw). Then combine the sounds into a long vibrating khhhhhhh. That is the 'kh' sound.
Loch (and Lach in
Lachlan) are prononced LOKH, not LOK, if you're Scottish.
However, here in Australia,
Lachlan is LOK-lan :-)