For fertilizing a hanging basket, or any ornamental growing in a container for that matter, I'd recommend buying yourself some Osmocote. I apply the required amount in my containers when I pot my ornamentals, then I throw some straw mulch on top and water it in well.
Done.
Now, for the entire season (in my case 4 months), everytime you water, your plants will be getting a slow-drip of N-P-K.
There are also balanced 14-14-14 versions of Osmocote w/ minor nutrients that are 6 month varieties and versions that are more geared towards ornamentals only. There are 3-4 month versions, 6 month versions, etc.
I got mine really cheap at Ollie's, not sure if they are Mass yet, for $2.99 a bottle instead of $6 or $7. They are selling the old-school traditional Osmocote, 19-6-12, in a 3-4 month release form. Set it and forget it!
And of course, if you are feeling like you want to be extra super loving and caring, you can always give the plants a 1/2 strength dose of Miracle-Gro liquid or 1/2 strength Dyna-Gro "Grow" liquid. I prefer Dyna-Gro as it has all the majors and minors is a liquid form. I use it 1/2 strength on containers that have been given the Osmocote treatment at the start of the season.
Just my 2 cents and the way I do it! Cheers! And yes, plants that get leggy, as the above posters mentioned, tend to not be getting enough direct overhead sunlight during the day in my limited experience.
