So you pick 25 Dominion cards that you have, and then play with those 25 as if they were a standalone - you play games using a random 10 out of those 25. You pick the 25 with some theme in mind, although if your theme is just "stuff I like" that's fine. Here are some examples.
Interaction Mix
2: Duchess, Embargo, Fool's Gold
3: Forager, Masquerade, Smugglers, Swindler, Trade Route, Watchtower
4: Bishop, Gardens, Noble Brigand, Throne Room, Tournament
5: City, Council Room, Cultist, Embassy, Governor, Graverobber, Jester, Knights, Tribute, Vault
6: Border Village
This one just focuses on player interaction. It's bound to be a pip, so I put it first.
Top o' the Deck Mix
2: Beggar, Courtyard, Herbalist, Vagrant
3: Lookout, Swindler, Watchtower, Wishing Well
4: Armory, Bureaucrat, Ironmonger, Nomad Camp, Scavenger, Scout, Spy, Walled Village, Wandering Minstrel
5: Ghost Ship, Graverobber, Knights, Mandarin, Mystic, Royal Seal, Stash, Treasury
Lots of cards interact with deck tops, and sometimes they naturally work together.
Hand Theme
2: Beggar, Crossroads, Hamlet
3: Hermit, Menagerie, Oasis, Tunnel, Warehouse, Watchtower
4: Fortress, Horse Traders, Remake, Tournament, Young Witch
5: Count, Embassy, Explorer, Horn of Plenty, Inn, Library, Mine, Pillage, Rabble
6: Hunting Grounds
8: Peddler
Cornucopia tried on a hand theme, but abandoned it because it's just invisible. The cards played well together though, so here's the kind of thing you can get with that theme.
Villageless
2: Courtyard, Embargo
3: Loan, Market Square, Wishing Well
4: Militia, Monument, Remodel, Spice Merchant, Spy, Trader
5: Apprentice, Count, Duke, Ill-Gotten Gains, Laboratory, Library, Merchant Ship, Rogue, Royal Seal, Upgrade, Vault, Venture
6: Fairgrounds, Hoard
Do you need a Village for a fun game? This set tries to argue otherwise. There was a lot of room for varying this one, since hey, most cards aren't Villages.
Big Alchemy
P: Vineyard, Transmute, Apothecary, Scrying Pool, University, Alchemist, Familiar, Philosopher's Stone, Golem, Possession
2: Cellar, Herbalist
3: Great Hall
4: Conspirator, Coppersmith, Ironworks, Procession, Wandering Minstrel
5: Apprentice, Haggler, Horn of Plenty, Inn, Jester, Library, Stables
Alchemy was once a large set. What might it look like as one? This isn't trying to look like the original large Alchemy, but rather just to push Alchemy's themes with the additional cards.