The ranger may make a periodic journey to a sacred location, such as a temple, mountain peak, or holy village. The pilgrimage may take place on a particular date (say, the first day of spring) or whenever he gets the chance within a particular timeframe (he must make the pilgrimage once a year).
Some faiths may impose strict prohibitions on the ranger's behavior. For example, the ranger may not be allowed to eat meat, wear head coverings, or start more than one fire in the same day.
Players and DMs may also make up their own religious requirements. Remember that a ranger might follow practices unlike other disciples of the same faith; just because most worshipers of a nature deity make shrines out of polished stone doesn't mean that a ranger can't build a shrine out of deer bones. Care should be taken to ensure that the practices don't conflict with the requirements of the ranger's character kit (it's unreasonable to expect a Sea Ranger to make an annual pilgrimage to the middle of a desert).
Most importantly, any routine practices should be simple enough that they don't distract from the campaign or tie up an undue amount of the ranger's time and effort. A ranger who has to come up with a weekly offering of 500 gp or must spend four hours per day in solitude won't be welcome in very many parties.
Once the practices are established, the ranger is expected to follow them. Should a ranger fail to uphold the requirements of the religion, or intentionally violate them, he may lose the use of his spells for a brief period, he may become ill, or one or more of his followers may desert him. The DM determines the penalty based on the severity of the ranger's negligence. In most cases, a warning should be enough (which the ranger may experience as a dream or vision), but continued abuse may call for a more severe response.
Rangers have much in common with druids. For instance, both classes are predominantly oriented toward nature. Both receive spells in similar ways; in fact, both of the ranger's spheres are shared with druids. Both strive to live up to the tenets of a higher power. They are natural allies against raiders, evil humanoids, and others who would despoil and ruin the land.
Some rangers won't form anything other than casual relationships with druids. They may work with them on projects of mutual interest and occasionally call on them for guidance or advice, but otherwise the rangers go their own way, oblivious to how druids exercise their faith and not particularly concerned with what goes on in their places of worship. In special circumstances, however, rangers and druids may establish more formal alliances, especially when a druid becomes a ranger's follower.
Any ranger may acquire a low-level druid as a follower, and may do so for a variety of reasons. If the ranger is skilled in areas in which the druid is deficient, the druid may seek him out as a teacher. The druid's superior may instruct him to serve a ranger as part of a penance, or to serve as a go-between for a network of wilderness protectors that exchanges information and favors. Or like other human followers, the druid may be drawn by the ranger's reputation, hoping for a working relationship that will allow the projects of both to prosper.