Even if it may seem like tuning a stringed instrument is a really easy process, in some cases it is more complicated than just turning the tuning pegs. What you want to accomplish by tuning your instrument is to make it sound right and to make it stay in tune for a long time. The better the instrument, the better the tuning pegs and therefore the more you stay in tune (but it is not just that). Even if locking tuner for examples help a lot in staying in tune, there are other factors like: the connection between the neck and the body (if it is not tight enough the strings will be easy to bend by pushing the neck forward), the action (the lower the action, the more force the string exercits on the tuner), the strings in the back (for electric instruments that use such springs, make sure that you find the right angle for each instrument to keep the metal claw at), the bridge (make sure that the springs there are tight and that the nuts are as powerful as possible), the nut (make sure it is treated with graphite) and really every place the strings go through (would be an idea to spray it all with graphite). The order in which I reccomand tuning is from the bridge to the neck and to the tuners. Going from the hardest to the easiest part will surely help you undo the tuning mistakes much more easily.