I posted directions for tuning a dipole on the SSCA Board a few years back. Here it is:
SSCA Discussion Board • View topic - How to Tune a Marine Dipole Antenna
I presume you're talking about a vertical dipole for marine use. If so, it will be considerably shorter than the formula, but it's best to begin at the formula lengths, then successively trim each end back by an inch or two and try again until you get the lowest swr.
An antenna analyzer would be a help, but you can do it with just an swr meter by tuning
quickly below the desired frequency to see where the antenna is "resonant" NB: resonance isn't the same as lowest SWR, technically, but for our purposes let's pretend they're the same.
You'll be tuning
below the desired (formula-cut) frequency because you know that the antenna is too long. So, blow softly into the mic and tune
quickly down the bank and watch the swr go down. If it begins to go back up again, you'll know you've passed the point of "resonance".
As you get the antenna cut so it's now "resonant" within the desired ham band, then cut the ends VERY LITTLE....a half-inch or an inch at a time...until you bring it up to the desired frequency.
Be sure to have a bottle of Mt. Gay handy. This is a tedious process without an antenna analyzer, but the Mt. Gay helps to make it fun :-)
Bill