Most Popular
If you want to use the most popular Rammus Support build, you should take Flash and Ignite as your Summoner Spells. For runes, you should pick Aftershock as your keystone, followed by Font of Life, Conditioning and Unflinching in the primary tree, as well as Legend: Alacrity and Triumph in the secondary tree. For rune shards, you want Attack Speed in offense, Health Scaling in flex and Health Scaling in defense, though it depends on the matchup. Finally, for your items, you should build Celestial Opposition as your first item, followed by Plated Steelcaps and Thornmail. For your last 3 items, you should build Jak'Sho, The Protean, Sunfire Aegis and finish with Vigilant Wardstone. For the skill order, you want to stick with E - W - Q. Right now, this build has 49.7% winrate over 350 matches played.
Alternative
If you're looking for alternatives, you can use Flash and Ignite as your Summoner Spells, with runes being Aftershock as your keystone, Demolish, Conditioning and Unflinching in the primary tree, and Celerity with Waterwalking in the secondary tree, as well as Attack Speed, Health Scaling, and Health Scaling as your rune shards. The items that this build incorporates are Solstice Sleigh as your first item, then Mercury's Treads as a second item and Thornmail as a third item. You want to finish the build with Dead Man's Plate, Hollow Radiance and Frozen Heart. As far as ability max order goes, you want to keep it W - Q - E. This build is currently sitting at a 49.4% win rate across 162 matches played.
Off-Meta
If you want to try an Off-Meta Rammus Support build in the current patch, try Ignite and Barrier as your Summoner Spells, Phase Rush, Nimbus Cloak, Celerity and Waterwalking as your primary rune tree, and Relentless Hunter with Zombie Ward as your secondary tree. Use Attack Speed, Move Speed and Tenacity and Slow Resist as your shards. For items in this build, you want to complete Celestial Opposition as your first item, then finish Boots of Swiftness as your second item, and Dead Man's Plate as third item. The last 3 options are Force of Nature, Thornmail and Warmog's Armor. The ability order for this build is Q - E - W. As of the current patch, this build has a 52.8% win rate over 72 matches played.
Situational Rammus Runes and Items
Depending on the context of your game, your matchup, and enemy champions, you might want to consider building some situational items or take specific runes. Carefully examine your team, enemy team, game state , and your matchup to plan your Rammus build accordingly! For example, when it comes to rune shards, you might want to take either Armor or Magic Resist, depending on which champion you are about to lane against.
Furthermore, if you want a more detailed breakdown, you should also check out Rammus runes page.