Hardy-Weinberg and allele frequencies
The frequency of the recessive phenotype equals q squared, so take its square root to get the recessive allele frequency q.
A worked example
If 9 of every 100 snails show the recessive shell colour, then q squared is 0.09, so q = 0.3 and p = 1 - 0.3 = 0.7.
The mistake people actually make
q squared is the share of individuals showing the trait, not the share of alleles. The model also assumes a large population, random mating, no selection and no net migration.