Photoshop basic tips

How to rotate the canvas?
Image/Image Rotation/

How to unlock the background?
Double click on the layer, OK, or
Right click, layer from background, OK

How to adjust levels (to make the drawing lines darker)?
Image/Adjustments/Levels (Control L), the move the black INPUT arrow to the right.

How to set the drawing layer to multiply blend mode (to see through the white)?
See where it says Normal on the layers palette? Click on the word Normal and select Multiply from the pop-up.

How to lock a layer?
Select the layer (so it is blue) and hit the lock icon in layers.

How to make selections, add to the selection, subtract from the selection?
Use the polygonal selection tool.
Do not use the magnetic or regular lasso tool.
The lasso tools are 3 from the top on the tool palette.
If the lasso does not have straight lines, press on the hidden tool triangle and select the right tool, the polygonal lasso.
The property bar under the main menu changes with each tool. Look for 4 small boxes to the left.
Experiment with using these modes: new, add, and subtract.

How to fill a selection with a color?
You need a selection first. Hover over the selection and right click/Fill.
Or, go Edit/fill (hit Shift F5)
Or, hit Alt backspace to fill with the foreground color.

How to change a color?
Image/Adjustments/Hue Saturation (Control U)