The wiki has most of it, but the best place to look is the code.
int64 nActualTimespan = pindexLast->GetBlockTime() - pindexFirst->GetBlockTime();
printf(" nActualTimespan = %"PRI64d" before bounds\n", nActualTimespan);
if (nActualTimespan < nTargetTimespan/4)
nActualTimespan = nTargetTimespan/4;
if (nActualTimespan > nTargetTimespan*4)
nActualTimespan = nTargetTimespan*4;
// Retarget
CBigNum bnNew;
bnNew.SetCompact(pindexLast->nBits);
bnNew *= nActualTimespan;
bnNew /= nTargetTimespan;
if (bnNew > bnProofOfWorkLimit)
bnNew = bnProofOfWorkLimit;