Euston's taxi rank is in the basement of the station effectively, so very close! On the concourse, near the entrance to the Underground, there are stairs that go down to the taxi rank - it is well signed. 1 minute walk down the stairs, along a short passageway and then it's on the left.
Kings Cross' taxi rank is to the west of the station, just round the side on Pancras Road, but is essentially right outside the west side of the station where the new dome is. 1-2 minute walk really from the arrivals at the front of the station to just outside the west concourse.
Neither are very far and well signed.