Francois, Antonio, Patrick, Richard, and Dainton Wu are all unlocked by playing through each of the 5 restaurants. Pretty straightforward and easy to get!

Jimmy and Emily are unlocked by collecting silver coins, e.g. the coins you get from customers when they order something that they like. If you're having trouble with this one, make sure that you're choosing menu items that correspond with the customer forecast in each level.

Angela is unlocked by getting three stars in every level. This can be difficult; I recommend again making sure that your menu items are chosen correctly. You can also prioritize certain customers over others, for example I usually give kids less items because they tip lower than other customers. I also like to keep items that take longer to make in my tray, that way I can get them to customers more quickly, although this may not be feasible if you don't have much tray space. Good Luck!

Julie, the Violinist is unlocked by buying every upgrade in every restaurant. She goes hand in hand with Angela, in that you won't be able to unlock every upgrade before you get 3 stars in every level. since you get money from getting stars.

Carl, the Mouse is unlocked by getting every mouse. This isn't too hard, thankfully, and if you're having trouble, Minoushje has made an excellent guide here on steam:
https://steamproxy-script.pipiskins.com/sharedfiles/filedetails/?id=805389053

Snuggy and Rosie are unlocked by collecting every memorable moment in the game. Memorable moments are those times when a camera pops up, and if you watch all the cutscenes you should probably get this achievement no problem. However, if you missed one, it can be frustrating to figure out where you need to go to find it, so here's a list of every level that has a memorable moment in it:
1-6 1-8 1-9
2-3 2-8 2-9
3-2 3-3 3-4
4-2 4-4 4-8
5-3 5-4 5-9

Brigid is unlocked by using every menu item. This is easier than it sounds: simply go into the last level of each restaurant, choose every item from the left side, start the level, restart as soon as the game lets you, and repeat except choosing every item on the right side.

The Soccer Moms are unlocked by letting Emily walk 15 miles. You should get this one pretty quickly, just by playing through the game.

Evelyn is unlocked by cleaning a table while customers are waiting for it. You might get this one just by playing, but if you're having trouble, play one of the earlier levels and don't clean any tables until a group of people comes in.

The Foodies are unlocked by getting a 3-customer bonus twice in a row. This means checking out 3 customers at the same time, twice. This isn't too hard, simply wait until you have enough customers lined up before you check them out.

Elvis is unlocked by using your register 1000 times. If you're having trouble with this one, try to check out customers one by one instead of in groups of two or three. Your score may suffer, but it'll still count towards Elvis even if you don't complete the level.

Sid, the Cleaner is unlocked by burning 10 dishes and still getting 3 stars. I got this achievement in level 2-1, by always having a pizza in the oven, however that won't work if you have the oven upgrade. I'd recommend using the sub sandwich buns in the 3rd restaurant for this, but try different things and see what works for you.

Edward is unlocked by clicking on the door in the top left of the second restaurant 4 times.

Kate is unlocked by finding 10 hidden objects, such as the blocks in the first restaurant and the potato head pieces in the fifth restaurant. Unfortunately replaying the level and finding the objects again won't count towards this achievement, they have to be unique items!

The Ducks are unlocked by collecting every other achievement, although they seem a bit buggy; I managed to unlock them before I unlocked Elvis.

I hope this guide was helpful, thanks for reading!