You should be fairly a experience skater before you try this. (Like: Ollie 3 decks, 50-50 rails…)
Come at your target (easier to learn on rails) at a speed that you can grind at and a speed that you are still comfortable with. Pop an Ollie and get above the rail (like in the 3rd photo below) and land on your tail almost like a salad. When you land you should have all your weight at the back truck so your nose will quickly move down and then you it will lock your board into place (Picture 4). Now even out your weight so your nose doesn't shift down, your weight should be spread out much like a 50-50 grind. Then grind across until you get to the end and just shift your weight back to your back truck to even out your board and ride out.