Take a bootstrap sample of each sample - a random sample taken with replacement from each of the original samples, of the same size as each of the original samples.

Calculate the bootstrap statistic - a statistic such as difference in means, medians, proportion, etc. computed based on the bootstrap samples.

Repeat steps (1) and (2) many times to create a bootstrap distribution - a distribution of bootstrap statistics.

Calculate the interval using the percentile or the standard error method.