Just use distilled/bottled water, and mix it up like you would any other shot.. Don't worry about the saline. Just mix dose of K with 50-75 units of water, filter with cotton (at least K is typically pretty pure), bang away.
If you have never IVed a drug before, then let us know and there is some other pertinent info and threads which are mandatory before doing it. That is if you are interested in safety.