Wats it called when you write hirigana/katakana by hand to spell out phoenetical english?

I just finished learning hirigana but i don't know any Japanese vocabulary.
So i have a dilemma because the main reason i started learning Japanese was so I could write private journals no one else could read(and i like the way it looks among other things).
So my first question is when writing Japanese characters is it an established practice to use Japanese characters to write English phonetically?
Is it advisable to learn it like this?
And what is the fastest/most efficient way to write to myself by hand in the long run? actual hirigana and katakana or using romaji?