Here's the scenario: I was a student pilot on inbound for landing at my home airport on my final solo cross country flight I needed before doing a checkride.
At roughly 8 miles out to the north west, I established contact with the tower and was told to "enter left base, runway 7, report 3 miles out." I interpreted the "report 3 miles out" as to give the tower a call when I was 3 miles away from the field, ie, when my GPS indicated I was 3 miles from the field.
Now, it was the most windy day I had flown solo in so far (15kt gusts) so I wanted to make sure I had ample time to line up with the runway on final and get a feeling for how the winds were blowing since the ATIS reported them as being variable in direction.
That said, what I ended up doing was making a wide base, except apparently I made it a little too wide because the GPS did not indicate I was 3 miles from the field until I was on final. At this point, I called the tower, reported I was on a 3 mile final and was given clearance to land. After landing, I still had no idea I had done anything incorrectly as the controller made no indication to me.
A few hours after I landed, my CFI sent me a text message asking me how "report a 3 mile base" meant "report a 3 mile final and fly a straight in approach." I'm not sure if he was on the frequency when it happened or if the tower controller called him after the fact and told him about it because I was told to "report 3 miles out", not "report 3 mile base."
I completely understand that I made my base leg too wide and that caused me to deviate from an ATC instruction. What I'm wondering is how bad of a screw up is this? My CFI's reaction was one of legitimate anger and it makes me want to stay on the ground for a while.