Cover six things. What shipping costs, including any threshold for free shipping. How long orders take to process before they ship, which is separate from transit time and is where most complaints originate. Estimated delivery windows by method. Where you ship to, and explicitly where you do not. What happens if a package arrives damaged or does not arrive at all. And whether you provide tracking, which at this point customers assume.
The processing time distinction is the one that causes the most trouble. A customer reading three to five days assumes delivery, while you meant time to pack it before the carrier collects. Stating both separately, in plain language, removes the single most common cause of an angry message on day four. Say that orders ship within a stated number of business days, then that transit takes a further stated range.
Be conservative with every number. A stated window you beat produces a pleased customer. One you miss produces a complaint, a support message, and occasionally a chargeback claiming the item never arrived. The temptation to advertise the fastest possible case is understandable and costs more than it earns.
Put it where it is visible before purchase rather than only in the footer, because a shipping cost discovered at the final step is among the most reliable causes of an abandoned cart. Link to it from the product page and show the cost in the cart as early as you can. Then keep it current, because a policy describing carriers or timeframes you no longer use is worse than none at all.
Decide what you do when something goes wrong before it does, and write that into the policy rather than deciding case by case. Whether you replace or refund a lost parcel, what evidence you require for damage, and how long somebody has to report a problem. Handling these consistently is what prevents a reasonable customer becoming an angry one, and having the position written means the decision is not being made while somebody is frustrated with you.
Review the numbers whenever your carrier or your process changes, because a shipping policy is the page most likely to become quietly inaccurate. Rates rise, services are discontinued, and processing times shift as volume grows. A policy promising something you no longer do is worse than none, since it creates an expectation you will fail and a document a customer can point at when you do.