There are two options. Summon an exist spiritual being or create a new spiritual being.
The first could be very dangerous. The second is dangerous too and very difficult.
The first is summon a demon for example, or a planetary (nature) spirit and ask him for help.
(Johannes Faust, Magia innaturalis, Arbatel: Of the Magic of the Ancients and others texts)
The second is create a new being by using a meditation.
(Franz Bardon - Initiation Into Hermetics, Stanislas de Guaita- La Clef de la Magie Noire an so on...)
Best way would be... lets use expression place an add or trow a bait and wait for a entity to take it(during meditation form a sphere of energy in which place a plea for assistance or help and the you wait once the entity responds you must treat it with respect, at ALL times, you must form a pact/deal/contract with it and determine price for its services, and that is it in a nut shell of course it`s not nearly as simple as that but to many things that could go wrong with it for me to risk placing detailed instructions out in the open, also entity you form agreement with will only do what is outlined in the contract and no more than that)
As for making one I must say that while probably a bit safer that summoning is still far from simple or safe(not to metion my knowledge on it is fairly limited)
No you are pretty accurate in your doubt that spirits can not be made/created,... BUT it is possible to create a spiritual/energy construct with the purpose of fulfilling the role of an actual spirit(they are inferior to the real deal in there abilities but they are also somewhat more reliable as they are incapable of interpreting your instructions in any other way than the one you "programed" in them).
As for making assumption that you do not "pay" spirit servants I would advice you to think again, cause we give them our energy, by allowing them to latch on and take it or through any other method of energy transfer.
And just one more thing Spiritual Servant is not necessarily an animal/humanoid spirit it can be just as easily a ball of light(and YES I am perfectly aware that spirits are for most part invisible, but they are also for most part shapeless and due to that little fact also highly adaptable in the way they can manifest them self)