In a bit more detail:
All the mouse clicks are single left clicks.
Get into Surveyor's opening menu with its list of layouts.
Select the layout of your choice by left-clicking it once. Things are slightly simpler if you choose a layout with no sessions, in other words one with a zero in the square brackets after its name.
Click the "Load" button near the bottom right of the screen.
When the layout appears, move to the place where you want to put a train.
Click the bottom tab on the righthand side of the screen, the one with a spoked wheel.
When the tab is pulled out, check that the lefthand of the two larger circles near the top of the tab is highlighted. This should already be the case (in other words, it is the default).
Select your desired locomotive from the list by left-clicking it once. The list can be scrolled up and down.
Check that the circle with a plus sign inside it is highlighted. This is the default and therefore should already be the case.
On the main part of the screen, outside the tab, left-click once on the track and the loco should appear there. To avoid complications keep away from junctions.
Add more wagons or passenger carriages as desired behind the loco by selecting them from the list, then clicking the track next to the train.
Then move the mouse up to the top left of the screen. A new set of menu option buttons will appear. Click the "Quick Drive" button.
When asked whether to save, do so.
When asked whether to create a new session called "Default", do so by clicking the green tick.
You should then be asked whether you want to drive in CAB mode (more realistic, more difficult) or in DCC mode (less realistic, less difficult). Click whichever button you want.
Then you should be able to drive the train.
There are several ways of exiting back to Surveyor, for example press the ESC button on the keyboard.
Hope this gets you going. Sorry if it tells you a lot that you already know, I was trying to list everything that you need to do.
John