Right now, I am a vocalist and bassist. I have an acoustic guitar and a keyboard, and I want to become mildly proficient in both of these instruments, but I am not really sure where to start with either. I'm especially at a loss with the keyboard, as I haven't seriously touched one in five years (two years of "lessons"). I do already have a decent knowledge of music theory, as well.

So basically, I am asking anyone who has learned these instruments how they would recommend starting.

In short, practice. I know it is a generic answer but it is true.

With two years of lessons I am sure you surpass me on keyboard ability, but I would suggest the usual of picking up songs you like. Start with easier ones and working your way up from there. Try to remember that depending on what subgenres of metal you are into the guitar will more or else be the melody which is much less true for bass that you are used to playing, it will be more like the vocals. Metal keyboard ranges from simple accompaniment to ridiculous leads so obviously you would want to start with backing stuff at first and even look outside of metal for songs to work on.

Learning all instruments are essentially the same in my opinion, start slow, learn the proper techniques, and practice forever. Making it fun is the best way to ensure that you keep up with the practice, which is why I suggest trying song you like. On the theory side, keep in mind that in addition to chord voicings, inversions, etc. that on guitar you can play the same notes in different positions and will get slightly different tonal qualities out of them because of the varying string thickness. At least one person I know got hung up on this when coming to the instrument with a theory background and will see an e on sheet music and not be sure where to play it.

Search the forums for threads on easy _____ metal songs to play, depending on what styles you are into.