If you shape the Atlas into only giving you the maps you want, you will get that map drop each time a map from that tier drops. So, if you have not played any tier 11 maps except for Shaped Strand, you will only get Shaped Strand Maps from monsters if that tier drops. However, if you play on any other tier 11 map as well, any of the ones you've visited has a chance to drop.
It is very important to remember that you do not need to complete the bonus objective to unlock the spot in the Atlas. Just once going to the map will unlock it as a potential drop.
Before you begin, you should run all the tier 1 to tier 9 maps to unlock those. Make sure to complete the bonus objective to get as high of a bonus as you are able to.
Tier 10: Run Collonade to get a Shaper's Orb, so complete the bonus objective there. Use this Shaper's Orb on either Dunes or Mesa (depending on your preferences). Create a yellow Cartographer's Seal (Vendor Recipe: 3xJourneyman Cartographer's Sextant + 1 Orb of Scouring) to uncomplete Collonade. This will prevent further Collonade maps from dropping, as long as you don't enter it again.
Tier 11: For tier 11,
To get Shaped tier 11, buy an Excavation Map, upgrade it to blue with a Transmute, and reroll it to only have one mod. Then use a Regal Orb to add another mod and make it yellow. Once that is done, Vaal it and hope it don't get too crazy. If you are able to complete it, go ahead and do that, but the endboss at that map is pretty difficult for a lot of builds, so don't be discouraged if you need to buy another map and hope for better rolls.
Once you complete Excavation with the bonus objective, use the Shaper's Orb on Strand to get Shaped Strand as possible map drops. Now you need a Master Cartographer's Seal to uncomplete Excavation, but this time you need 3 x Master Cartographer's Sextant + 1 Orb of Scouring, so this one is a bit more expensive to uncomplete compared to the tier 10 ones.
Tier 12: Many people choose not to run or shape any tier 12 maps, simply because any tier 12 maps that drops will be downrolled to tier 11, thus getting you more Shaped Strand maps. However, some people also enjoy running Shaped Mud Pools or Ashen Woods, so that is okay if you like those maps. To do this, run a rare corrupted Estuary Map to get the Shaper's Orb. Once that is done, uncomplete the Estuary Map with the same recipe as tier 11.
Tier 13: Most people agree that Atoll is the best use of the Shaper's Orb, so this should be the one you Shape. To get the Shaper's Orb, either go for a rare corrupted High Gardens Map, then uncomplete it with the Master Cartographer's Seal. The other option is to complete Oba's Cursed Trove. I prefer the last one, because you don't need to worry about uncompleting it. This map is Unique, so it will drop pretty rarely, and it is worth around 3 Chaos anyway, so you can just sell it and but another Shaped Strand or Atoll map for those chaoses anyway.
Tier 14: The Shaper's Orb can be gotten from either a rare corrupted Volcano Map, Poorjoy's Asylum or Death and Taxes. Both the first two options are fine to keep completed, since they are worth a decent amount. If you plan on running Shaped Strand, you won't really see these drop anyway.
You can choose to Shape any map you want for this tier, but Coves, Reef or Overgrown Shrine are generally accepted as good choices.
Tier 15: You can get the Shaper's Orb for this tier from Overgrown Ruins (rare corrupted), The Vinktar Square (Unique) or Vaal Temple (rare corrupted). People generally agree too Shaping either Underground River or Quay for this tier, but feel free to do the one you like the best.
With this setup you should be good to run the best map for the most efficient farming. You can start out by running tier 9 maps to get a nice supply of Shaped Mesa maps and Shaped Strand Maps. These will give good experience as well as very good currency drops, depending on your luck of course. Once you run out of these, run any tier 12 or 13 until you get a refill, and start by running Mesa and Strand again.
This is generally the best setup for efficient farming, but you can change around until you get the ones you like the best. Running Shaped Strand all day can get pretty boring, so if you prefer fun over efficiency, go ahead and do a little variation.