Most of the above advice is very good, especially how you will need to rev the engine more than you first thought. Try it at 15 mph first. To give an idea how fast you should rev the engine with the clutch out and in neutral, leave the car in 2nd gear and take it to 15 mph. Now LISTEN TO THE MOTOR before you shift into top. That is how many revs you want (or slightly more) as you shift into 2nd on the down shift. The whole idea of double declutching is to match the speeds of the gears so they engage without grinding.
Once you haave read all this, go out and
practice