Really you don't need an actual spell but more an 'intent' to astral project. I stumbled upon a method myself just trying to project...
FIRST, you must shield yourself or cast a circle or whatever method you use. Now, second, try lying down on your bed, wear non-constricting clothing (pajamas or scrubs, that sort of thing) and just close your eyes and just 'be' for a few moments [meditate] then try to lay as still as possible, so still that you stop feeling your body [if you've ever done this in a pool as a kid then you are one step ahead] just concentrate on becoming still and clearing your thoughts... This took about 45 minutes to an hour the first few times I did this, but after practice it speeds up to 20-30 minutes... When you are still, usually you are dozing off to sleep but not deeply at all, as long as you try NOT to feel your body you shouldn't end up falling fully asleep but you end up in the 'gray zone' between consciousness and sleeping... then you will know you are getting close when you actually FEEL your entire body buzzing slightly, then it will intensify ~ this will awaken you only a little and you will start to actually hear it, it sounds like a TV channel with just static, keep concentrating a few for minutes until you 'POP' out, or your arms may 'pop' out first, then the rest of you... You might get nervous, excited, a slightly elevated heart rate, etc. the first few times because you're feeling it happen [which will prevent you from exiting fully so stay calm and surrender to it] ... When you do pop out DO NOT BE AFRAID, if you feel that there is someone else in the room, know that there is NOT, you are 'feeling' the presence of your own body near you. Becoming afraid will pull you right back in so now you know that there is nothing to fear around you.
Only one warning ~ The reason I say to shield yourself beforehand is so that you can keep from getting 'attachments' if you end up too close to the lower astral planes. Read a little more about that but don't give in to those articles that fear-monger... trust yourself and your shielding, it is safe to experiment.
ANYWAY ~ This is just MY method so it may or may not work for you but you can get a better explanation with a book dedicated to this subject, I have one in .pdf format so if you or anyone want to pm me I can email it to you, it's called 'the Astral Projection Workbook'.. check amazon for the book summary. Also, the book Astral Dynamics by Robert Bruce is a good one, too, but it is quite involved and pretty thick, yet still a good read [I only have a hard copy of that one, not digital, lol]
Good luck on your quest, I hope I've been helpful! :)