Pearldrums is absolutely correct, if your form varies, so will your results.
Pat has given very good general guidelines that will get you close, and then tune from there.
and as Knoll and John said, there are some great videos out there, here is a link ( at the bottom of the post)to the best one i have ever seen, i forward this to a lot of people. it was a real eye opener for me!!!
bareshafting is an awesome technique, its all i use, but it does have a few quirks though- which, until you get it figured out can be hair yanking material.
this is a pet topic of mine, so if i may, i shall expound on this a bit.
1. as Pearly said, form!! now you are not going to improve on your form, unless you shoot, and you cant shoot unless you have arrows,!! so what do you do... you do the best you can- you will know when you pulled a bad shot, and just ignore its results. shoot slowly, stay fresh, dont fatigue yourself, take lots of rests a tiny bit of fatigue will lead to changes in form.
2.they tell you for bareshafting to hold your bow nice and vertical- now that just never made sense to me, if i have spent my whole life shooting with my bow canted, and my form has developed around that stance- to change all of a sudden for a few shots, is more than likely going to wreak havoc with ones form, so to counter that, i tilt the target over to match the angle of my canting. i like to draw a vertical and a horizontal line on my paper target face, that helps me to evaluate my results more accurately. i get some one to stand behind me, and see if my canted bow matches the vertical line on the target, and i adjust the target till it matches the bow- not visa versa.
so when i step up to the target i can assess the results , ensuring that i am not interpreting a bit of nock high or low as incorrect spine. because i have both a horizontal and vertical plane to check the shafts against.
3. arrow breakages.
the first time i spine tested- (long before we were making shafts) and i only had a single fresh dozen shafts i had just purchased, i figured that the further i stood from the target the less energy the shaft would have when it impacted the target, and that would prevent any shafts from breaking- oh how wrong i was- i broke every single shaft, i started at 10 yards, broke some!! then backed up to twenty and broke some more, and broke my last few out at 30 yards. needless to say i was devastated!!!
i know its counter intuitive, but you need to start close, real close to the target, and i am talking 3' close!!!
the problem with distance from the target, is yes, the velocity and energy is less than that at the bow, but by the time the shaft gets to the target , its so way out of whack- having no fletching to help it at all, that its hitting the target at 45 degrees or worse, and that will break any wooden shaft.
when you are close, yes paradox is having an effect , but if you watch the video, you shoot, you trim, you check, then you step back to 5' , you shoot, you trim, you check, then you step back to 7' etc etc.
its a lot more laborious, but you will break a whole bunch less shafts.
finally, try and get as close as you can in spine before you even start, there are good charts out there, (i have some), and there is a TON of experience on this forum, that will give you a really close starting point
good luck!!
https://www.youtube.com/watch?v=IGOPiriLbcM