It's an easy transfer if you follow the signage. Take it slowly, read the signage.
I'm always astonished that people just don't read signs!
It helps to know whether your direction is north or south, east or west.
For Kings Cross to Green Park, you're going either South or West (depending on which line). THe Victoria Line to Green Park (light blue) is probably the quicker of the 2 lines that go through Green Park.
But also remember, London is a very crowded city. A lot of us are trying to get somewhere with a limited time, and it helps if everyone is aware of the shared space.
So if you need to stop to consult an app (the TFL app is great for mapping the Tube system & specific stations) step towards a wall. Don't stop in the middle of a space or passage or walkway! Don't stop in the archways onto platforms, or even worse, at the stops of stairs/escalators. Stand to the Right!!!
And if you are in a group, remember the maxim "Walk narrow or walk nimble" That is, don't walk four abreast in narrow passageways or walk 4 abreast slowly ... Walk single file, and if you get lost, step aside. Don't just stop.
And let people off before you try to get on - buses or Tube. Science has proved, this is quicker for everyone.
And have a great time - London is probably the most amazing city in the world.