I've found the key to befriending magpies is don't be too eager. When starting off just drop a bit of food (I give them unsalted peanuts still in the shell) and walk away. Don't even look back. Don't rush it, it'll take a few days, maybe longer, but eventually you can build up enough trust where they'll be braver and take food that's closer and closer to you. Patience is key. It also helps to stick to a schedule, leaving them food in the same place at the same time each day.

Also make sure what you're giving them is healthy for them (stay away from processed/salty food) and don't feed them too much! It's okay to supplement their diets and build friendship but you don't want them becoming reliant on you, they are still wild animals!