It's a lot harder than you probably think it is

Disassembling, cleaning, and reassembling a modern lens is a daunting task for anyone who does not have the proper training, experience, and tools. There are so many things that the novice is not even aware of that can go wrong that it's practically inevitable that one such error will make the lens unusable.

Once the owner has already disassembled the lens, most repair shops won't even think of accepting it. The ones that do will charge a steep premium because it will take them a lot longer to diagnose the secondary problem (whatever damage was done by the amateur repairer) as well as require extra steps to realign things that weren't properly marked, such as lens centering and tilt adjustments, during the teardown. That's assuming they are familiar enough with the lens model to know how it is supposed to go together without seeing it together before each part was removed. It's impossible, for example, to know that a focusing element position sensor of the kind used by many lenses has been contaminated by contact with human skin oils until the lens is reassembled and it is discovered that the AF has been bricked. At that point there's no way to tell the difference between a faulty ribbon cable and a contaminated focus position sensor. Unless you're at the lens factory that makes that lens which might have a custom tester that the focus position sensor can be plugged into, or the same for the specific ribbon cable in question, the only way to diagnose it is to start replacing parts one at a time, reassembling the lens, and testing. That can get very expensive very quickly.

So how is one to learn? Start with a lens that is already assumed to be a total loss. Buy a few specialized tools needed, such as precision JIS screwdrivers in sizes down to '000', spanner wrenches, and rubber cones. Get a general lens repair book, which should give you more specific guidance on what tools you'll need, and a service manual for the lens in question. Take it apart. Learn from the first several ribbon cables you break how to handle them so that they don't break. Etc.

If that sounds a bit much, there are a few other things you might try. Shake the lens around gently in different positions relative to the direction of gravity and see if the sand can be dislodged. Move the zoom and focusing rings from one end to the other, again with the lens in several different positions relative to gravity. Stop any movement if it feels rough or 'gritty'.

Your assumption may be correct about the diaphragm having a grain of sand in it, but it just as well could be that a grain of sand or some salt water spray has landed in the wrong place on one of your circuit boards and shorted it out.

Why this lens is a good candidate for being a 'total loss'

If you were at the beach and sand made it inside the lens, salt water spray/mist almost certainly did as well. There's little worse than salt water for the inside of a camera or lens. Most manufacturers will not even open a camera or lens up to repair it if there is evidence of salt water getting inside.

According to Roger Cicala, the founder and overall technical guru at lensrentals.com¹, saltwater damage is more devastating to cameras and their internals than just about anything the gear his company rents encounters on a regular basis. He covers it in depth in this blog entry, but he has also mentioned it in many others.

¹ Probably no one in the world oversees a larger inventory of cameras and lenses that are used to take photos, rather than being stored in a warehouse as inventory to be sold, than they do.

At Roger's company, they don't even part out unrepairable cameras with salt water damage due to the concern that there may hidden corrosion in those parts. Normally, those guys part out just about everything - even some full frame cameras with a single scratch on the sensor get parted out to repair other cameras in their vast inventory:

Lensrentals insider joke: What do you call a D800 with a scratched sensor?

Parts. Because at $1,800 for a sensor replacement . . .

But in the case of salt water damage:

But the amount of salt and corrosion here and on the bottom means we wouldn’t trust anything in this camera, ever again. It can’t even be a parts donor — the chance that those parts will eventually corrode and fail is too high. That’s why many service centers won’t repair water damaged cameras; they have to give a warranty after the repair and chances are very high something they didn’t replace is going to fail during the warranty period.