Also see ‘how to’ video above!
You can prepare these ahead, then take them along to your party. Roll up, wrap in plastic wrap and take along to your party like this. Then slice when you get there. Just don't prepare TOO far in advance. These certainly don't keep very well until the next day.
These are very easily adaptable according to what you've got in your fridge. Spead with cream cheese instead of the feta and yogurt mixture, if you like, then add chopped sundried tomatoes, bits of parma ham, grated cheese, fresh herbs, or whatever you like!
To be honest, the amounts stated in the recipe above are just a rough idea. Just use a small handful of everything and that will be about right.