Many people seem to have trouble rotating objects. There are hundreds of tutorials about rotating, but most of them don’t mention the trick that makes rotation extremely easy. Consider this image below. We have a plane in our drawing (the orange “line”). It’s not really a line (even though it looks like one); it is actually a flat object which we are looking at facing the edge. So we see the edge closest to us, but looking at it from this direction, we can’t see any depth.
To rotate it,
- You first select the object you want to rotate. (In this case, the orange plane)
- You make sure that the object origin is either in the middle of the “line” or on one of the ends. How to place the origin in the right place? Have a look at this article. For the moment, we make sure the cursor is in a good position because the plane will rotate about the cursor.
- Type
to initiate the Rotate command. - Move the mouse in the direction of the arrow shown until the plane rotates to the approximately correct orientation. It doesn’t have to be exact, just approximately. Then Left-Click anywhere in the viewport.
- A parameter panel will appear (usually in the left-hand bottom of the viewport). It might be the fill expanded panel as shown in the picture, or it might just appear as a heading like this:
. In which case, click the arrow to expand it. - In the Angle field, type in the exact angle it should be. Press
and then click into the viewport to complete the rotate operation.
. In which case, click the arrow to expand it.
and then click into the viewport to complete the rotate operation. 
So what’s the trick?
The trick is that when we rotate an object, we must be viewing the object to be rotated perpendicularly. In the picture above, we rotate the plane with complete control because we are looking at it orthogonally from the side view.
Consider this slab:
Now let’s say we want to rotate it so that the long side goes upwards. We need to choose which orthogonal view we can rotate it in.
If we try it in top view 
this is what we get:
So let’s try rotating the slab in the front view 
:
So we are left with rotating in side view 
:
When we then jump pack to perspective view, this is the result of rotating in the side view:
Another Example
Let’s look at another example in more detail. This time we will use a cone as our object:
Now let’s say we want to rotate this cone 90 degrees so that the flat base is now vertical. So, without changing the view in the picture above, we select the cone and press 
to rotate it. We rotate it using the mouse until it is approximately in position and Left-Click. The parameters panel for the Rotate command appears:
The resulting cone looks like this:
Well, that looks good, doesn’t it?
Err… Actually not!
If you look at our cone in ortho view by pressing , we see this:
Oops… So what happened to our 90 degrees? Well, in fact, Blender did rotate the cone 90 degrees, but it was done in a non-orthogonal view, so the cone was actually rotated partially on one axis and partially on another. Blender acted as if our perspective view were an orthogonal view. The result is a mess!
Can we fix it? Not easily. We will just have to undo 
that rotation and rotate it properly.
OK, so how do we rotate properly? By rotating in an orthogonal view ONLY.
So we undo our rotation and the cone is once again in the upright position. Switch to the orthogonal view like this:
Now in this Orthogonal view we rotate the cone, and in the parameters panel we type -90 degrees. This is what we get:
That’s better! The flat base is now vertical as intended. Here is a perspective view of the correctly rotated cone:
The only time you don’t rotate in orthogonal view is when you want to rotate an object in a diagonal plane or across the face of an object. In which case, select the face in Edit mode and set the view to be orthogonal to that face with 
. Or, if you prefer, you can use the Align View drop-down menu:
