If you are on a budget and in need of a cheap practice bass, you can convert an electric guitar to a bass cheaply and quickly. Although a bass furnished from an electric guitar will not be appropriate for use as a performance instrument, it can be used as a practice instrument, and if special care is taken with the wiring, can even be used with a bass amplifier. The process, however, requires a basic knowledge of electric guitar wiring, and involves modifications that require a lot of attention to detail.
Remove the neck and bridge from your guitar. Although it is possible to fix bass strings onto a set neck guitar by widening the nut grooves on the head stock, an instrument fashioned this way will not sound like a bass guitar. Getting the right amount of tension in the strings requires a longer neck designed for a bass.
Attach a bass neck onto your guitar body. If you don't have a bass neck, you can buy a used one at a pawn shop, or online through a guitar parts specialty store (see Resources, below).
Attach a bass bridge onto your guitar body. Depending on the type of guitar body you have, this might be more difficult than attaching the bass neck, because while a bolt on a bass neck should fit easily onto the body of a guitar by the same manufacturer, a new bridge might need to be glued on or bolted on with a new set of holes drilled into the body (see Resources).
Switch the tone on your guitar down so that the resonant frequency is around the 2,000 hz range. Although most electric guitars are wired differently from electric basses, if you set the tone of your guitar down to the low end after reconfiguring it with bass strings and neck, you will get a bass sound when you run it through a bass practice amp.
Attach bass strings to the guitar. If you have installed the neck and bridge properly, this should be the same process as stringing any normal bass guitar.
Experiment with different tone settings and cables until you get a nice, warm sound through your practice amp. Though you won't get a performance-quality sound out of a bass made by this method, you can get a loud and clear-enough sound to practice with.
Note that the process used here will most likely void the warranty for your guitar if you have one.