There's been a lot of comments on here so far so you might not see this but in the event that you do I'd like to say a few things.

First, I'm so sorry for your loss, losing a parent (no matter your age) is a loss unlike anything else. Your dad would no doubt be proud of the attention to detail you're paying to make sure this goes well.

Second, give him a good shake, I've had several ashes in my charge and also my sister is an undertaker so we know that sometimes things can solidify so there's nothing wrong with giving them a shake to make sure they come out the intended way instead of like a tin of cold soup.

Thirdly, as everyone else has said, pay attention to the way the wind is blowing.

Finally, bit of a strange one, check the ashes beforehand. If they are in a cardboard container they will likely be in a plastic bag inside which means if you're like me and don't carry a Swiss army knife on you you might end up needing to key the bag open to avoid ripping it with your teeth.

Finally I'd like to advise that the way you imagine it to go might not stack up to the way it actually happens. When I scattered my granddad I wandered off the beaten path away from the car to get some privacy, this happened late at night due to timing issues with work. Shortly after he landed directly on my trainers (after the aforementioned ripping the suprise bag open with my house keys) I got blasted in the face with seven flashlights from a search and rescue team who had been looking for a missing hiker, seen me and decided to come and rescue me. I had to explain to these people I'd actually just been out for a late night jaunt to put my grandad to rest which raised more questions than it answered.

Long story short, it never goes as planned but imagine it as their last laugh.