Your alt will need to run past 85-86 level mobs to get started farming. I'm not sure if you can summon a lower level toon to Pandaria, but Farmer Yoon's quests to start up your farm are level 85 and above, so your toon will need to be at least 85.
First, your alt will have to get to Pandaria through your faction's quest. You'll need to complete the vehicle quest, whether that's shooting Alliance troops from a turret or shooting Horde troops from a helicopter. Then, you should be able to drop to the ground via a rope and continue the rest of the way to Halfhill on mounted foot.
Both factions need to run for the main road on the Jade Forest map, then follow it until they can go west across the bridge into the Valley of the Four Winds. I'd advise Horde to avoid Grookin Hill and instead go around the longer way, unless you've gone beyond the minimum questing and made the Hozen friendly to you. Alliance will need to run through Nectarbreeze Orchard, but any mobs your attract should be outpaced by your mount.
Once you hit Valley of the Four Winds, grab the flightpoint at Pang's Stead, and then head west along the road until you reach Halfhill. Grab Halfhill's flightpoint, and then talk to Farmer Yoon to start the farm quests.
A Helping Hand introduces you to how your farm might eventually grow in plots, clearing away the boulders. You'll then be introduced to the Tillers Union through a roleplay scene when Haohan Mudclaw comes to see what Farmer Yoon is trying to do.
Farmer Yoon will teach you to go to Merchant Greenfield for seeds. Greenfield will later be a vendor, but he'll give you some special cabbage seeds through the dialogue options this time.
Farmer Yoon next teaches you how to till and plant your crops. Just right-click on the Untilled Soil to till it. Target the Tilled Soil and use your seeds in your bags to plant a seed in a plot.
Farmer Yoon will give you a Rusty Watering Can to use for this exercise. You can find the other tools at the farm just next to the hut and stump: a shovel for removing growing plants, a bug sprayer, and the watering can should you ever delete it from your inventory or forget to take it from your bank.
The special cabbage you just planted grows quickly, as Farmer Yoon tells you. They'll ripen right there before your eyes, but normal vegetables will ripen at midnight server time. (Remember, your server time can be different from your personal time!)
Finally, Farmer Yoon introduces you to the patches of Dark Soil everywhere throughout Pandaria that yield various shiny objects you can use for personal Tillers NPC reputation. This time, you're going to give a Marsh Lily to Gina Mudclaw, which is her favorite!
At this point, you can continue with Gina's Vote, or you can enjoy your 4-plot alt farm any way that you want. May your harvests be bountiful!
Mists of Pandaria is here! The level cap has been raised to 90, many players have returned to Azeroth, and pet battles are taking the world by storm. Keep an eye out for all of the latest news, and check out our comprehensive guide to Mists of Pandaria for everything you'll ever need to know.