i dont know how much you need but the easiest and best way is to buy john innes composts.
these come in numbers 1,2, and 3, these refering to various types of content.of mix.
john innes is in fact a formulae and when made has also been sterilised, very important.
at this time of year you will be able to buy some bags on offer, bowers is the best brand.
i am not knocking making your own but j. innes is the real deal. look up on google if interested.
trust this helps.