OK, so for your pots.
They need to be a decent size, otherwise they dry up too quickly
They need to be filled with compost or soil compost mix
Chose cheap and cheerful bedding plants, petunias are good as they like sun, and do well in post
Plant them, and give them a good water, the whole post should be wet through.
Then, and this is the hard but, you must water them every single day, even if it is raining, as a pot full of flowers doesn't allow much water to the roots.
Don't underestimate how much water either, water from the top until some runs out underneath. (all pots should have holed in the bottom)
That should work! Water int he morning or evening, not the heat of the day too.
If you snip off the dead heads they will go on until the first frost.
If your plants 'disappear' while they are small and you are left with just stalks, that is snails eating them, and some snail pellets, or standing them on sharp gravel will help