Once in a while, I will get someone emailing me asking about how to do a magic spell from my book or website correctly. "Should I do this first or that first? How much of this should I use?
First thing: read instructions, and if you are unfamiliar with casting your own spells or with the tradition of magic you're reading about, abandon preconceived notions. Usually just doing what the instructions say is adequate. Example: if the instructions don't say to chant magic words, assume there are no magic words to be chanted.
There's a reputation that magicians deliberately leave out information to "protect their secrets" but I really find that that is pretty rare -- a person who doesn't want to share their secrets usually uses the far superior method of not offering to share anything. There are basically 2 reasons (barring the occasional "author forgot to write it in") that a written spell might leave out information: 1. It's unimportant to the operation. 2. It's considered so basic it's not perceived as necessary to describe.
In the first case, things like quantities are often left out. For instance, it doesn't really make a difference whether you use a couple drops of oil or a whole bottle in most cases -- that sort of thing tends to be a matter of preference. Whatever strikes you as reasonable, is usually fine.
In the second case, there are bits of information that it's usually assumed a person familiar with the magic will be able to do on their own. Kind of like most cookbooks assume a certain amount of knowledge of how kitchens and food work -- they don't include steps like "use only clean dishes" or "cut open the package of chocolate chips with scissors or tear it open with your hands, but do not beat it with a hammer nor use power tools or creme brulee torches to open it" because it's assumed you will be familiar with these ideas and not require telling. Similarly, some spellbooks might assume you know certain things about conducting magic and so won't elaborate in a way perceived by the author as being unneeded -- they might assume you know where to get supplies, how to dress candles and burn incense, how to dispose of a spell and so forth.
However, there is something to remember that is the most important thing of all: If you can do it with total confidence, it's probably just as good done "wrong" as right. This phenomenon is often seen in incorrect conjure formulas being sold that get results just as good as the correct and traditional formulas. However, for this to work you must absolutely not second-guess what you've done -- you cannot come back asking your mentor "Even though it said to do that, I did this instead, is it okay?" NO! If you have to ask, then it's not okay; but you can get away with a lot if you really don't know better and you really believe you're doing right. Ignorance offers much power -- and so it is bliss.