The following is a brief description of how to unlock each ending in the game. These are not the only ways to finish each route, but I tried to pick an order of events that's satisfying for each one. You can play as few or as many as you like - There is no special reward for completing each one, other than seeing the content of that route.

This contains spoilers, so if you want to avoid those, simply go explore the game at your own pace!

During the prologue, you can select any name and pronouns you like. It will not limit your choices in any way.

1. The platonic ending

This is the easiest one to get, as it's also the default ending you'll receive if asking someone out on a date fails. Simply spend the ingame week as you wish, then when you are asked to choose a potential date for Midsummer, select the “No Date” option. You'll have a great evening with the friends you made, learn some extra tidbits of information and the ending screen shows a sketch of all the main characters.

2. Blossom's route

During the prologue, choose “Nice work, your animals seem really happy”during the talk with Blossom.
After the prologue, choose to hang out with Blossom and you'll get to help her with finding a missing rabbit. Choose “I'll help you look for him” and then “Carefully beckon him over” to finish the scene successfully.
On the next day, pick Blossom again and you will receive a pair event in which the two of you help out Moth in the tailorshop. Choose “I could help you out” and later “Helping out friends in a pinch is just the right thing to do” to finish the scene successfully.
Spend the remaining 3 days however you like, and then when Midsummer comes around, pick Blossom as your date to see her ending.

3. Moth's route

During the prologue, choose “Say Hello”. Whether you encourage him to call you “luv” doesn't have any impact on the relationship score, so just choose whichever you like!
After the prologue, choose to hang out with Moth, and you'll get to go on a spooky treasure hunt with him. Choose “This is really fun, but we should be careful” and then “Let Moth go first” to finish the scene successfully.
On the next day, pick Moth again and you will receive a pair event in which you help him out in the tailorshop. Choose “I could help you out” and later “Helping out friends is just the right thing to do” to finish the scene successfully.
Spend the remaining 3 days however you like, and then when Midsummer comes around, pick Moth as your date to see his ending.

4. Aleth's route

During the prologue, choose “It's okay, I get nervous talking to new people too”during the talk with Aleth.
After the prologue, choose to hang out with Aleth, and you'll get to save them from a tense encounter in the forest. Choose “Want me to help you with collecting more things?” to finish the scene successfully.
On the next day, pick Aleth again and you will get to mediate between Kaileen and Aleth. Choose “You know her best, what do you think?” and “Aleth needs your friendship more than a potion” to finish the scene successfully.
Spend the remaining 3 days however you like, and then when Midsummer comes around, pick Aleth as your date to see their ending.

5. Kaileen's route

During the prologue, choose “Now I know who to come to when I need help” during the talk with Kaileen.
After the prologue, choose to hang out with Kaileen, and you'll join her on a forest excursion. Choose “Nice, what will you be doing with that?” or “I'll just let her finish her work in peace” to finish the scene successfully.
On the next day, pick Kaileen again and you will get to mediate between Kaileen and Aleth. Choose “You know her best, what do you think?” and “Aleth needs your friendship more than a potion” to finish the scene successfully.
Spend the remaining 3 days however you like, and then when Midsummer comes around, pick Kaileen as your date to see her ending.

6. Nord's route

Nord and Atlas are special cases because they are already in a relationship with each other – but you can become part of this through the poly ending! For this, you will need a good score with both him and Atlas. The instructions for Nord's and Atlas' route are very similar, they only differ in a detail near the end.
During the prologue, choose “Thanks for letting me tag along!” for Nord and “Looks like you two got your routines all figured out!” during the talk with Atlas.
After the prologue, choose to hang out with Nord, and you'll get to help him during a tense talk with his boss Oberon. Choose “Don't yell at him, it's not his fault” or “It may have already been broken when it was packaged” to finish the scene successfully.
On the next day, spend time with Atlas. Choose “It's great that you guys take care of each other” and then “Yes!” to finish the scene successfully.
On the next day, pick Nord again and you will go on a scouting tour with him and Atlas. Choose “Let's go get him back”, then any of the options in the troll camp (it does not matter to the relationship score which one you pick), then “It's alright to depend on others sometimes” to finish the scene successfully.
Spend the remaining days however you like, and then when Midsummer comes around, pick Nord as your date. When he asks if you'd like to go on the date with him and Atlas, choose “I'd love to be the third in your bond!” to see Nord's ending.

7. Atlas' route

Nord and Atlas are special cases because they are already in a relationship with each other – but you can become part of this through the poly ending! For this, you will need a good score with both him and Atlas. The instructions for Nord's and Atlas' route are very similar, they only differ in a detail near the end.
During the prologue, choose “Thanks for letting me tag along!” for Nord and “Looks like you two got your routines all figured out!” during the talk with Atlas. After the prologue, choose to hang out with Nord, and you'll get to help him during a tense talk with his boss Oberon. Choose “Don't yell at him, it's not his fault” or “It may have already been broken when it was packaged” to finish the scene successfully.
On the next day, spend time with Atlas. Choose “It's great that you guys take care of each other” and then “Yes!” to finish the scene successfully.
On the next day, pick Atlas again and you will go on a scouting tour with him and Nord. Choose “Let's go get him back”, then any of the options in the troll camp (it does not matter to the relationship score which one you pick), then “It's alright to depend on others sometimes” to finish the scene successfully.
Spend the remaining days however you like, and then when Midsummer comes around, pick Atlas as your date. When he asks if you'd like to go on the date with him and Nord, choose “I'd love to be the third in your bond!” to see Atlas's ending.