My Time At Portia

Edit: Thanks to a comment from matoy13 it has come to my attention that you can actually get this achievement by missing a play date instead of an actual date. This means that you can do this as soon as your relationship is increased to buddy status, and it is not necessary to confess your love by given a Heart Knot. I will leave up my original guide below, as you can still use it to increase your relationship and potentially get progress for other achievements.

Short version:
1) Increase your relationship with a romanceable character to friend status (or further to increase your chance for step 2)
2) Confess your love by gifting them the item Heart Knot, which can be bought from Alice's shop. Rejection chance becomes lower the higher your relationship status.
3) Invite this character for a date but don't show up.
4) Reload save (optional)
Tip: Do this for Phyllis or Gust so the relationship points you invest can also be used for other achievements.

More detail:
To get this achievement you first need to increase your relationship status with a romanceable character. On the social tab of the menu you can see the status of all relationships in the form of a number of hearts/stars. The characters you can date have hearts next to their character portrait (except Merlin, which cannot be romanced).
The list of romanceable characters is:
Bachelors: Aadit, Albert, Antoine, Arlo, Django, Erwa, Gust, Liuwa, Mint, Oaks, Paulie, Qiwa, Remington, Sanwa, Siwa, Tody, Wuwa, Xu
Bachelorettes: Alice, Emily, Ginger, Lucy, Mei, Nora, Petra, Phyllis, Sam, Sonia
You can do this with everyone on this list, but if you are going for all achievements, you can save yourself some time by choosing Phyllis and/or Gust. Both characters need to be romanced for other achievements (Dream Come True for Phyllis and Romantic Proposal for Gust), so you can work on multiple achievements at the same time.

To date someone from this list, you need to increase your relationship level with them to four hearts, meaning you are now friends.
There are several ways to get relationship points:
1) Social interaction, e.g. chat, play rock-paper-scissors, spar (daily)
2) Gifting (daily)
3) Playing (once a week)
4) Completing missions/commissions from that character (when available)
5) Increasing relationship status with other characters (when applicable)
6) Displaying relics in museum/yard (random)
You can use your skill points on specific social skills to increase the number of relationship points given. For example, the smooth talk skill gives a chance for an extra relationship point for chatting.

Personally, I focused mainly on daily chatting and gifting their favorite gifts and I would check the commission board each workday for any commission available for that person. The other interactions just take too much time in my opinion and are not worth the bother. Also, go play with them once a week to increase your relationship even faster.
If you are romancing Phyllis and/or Gust, I recommend using these gifts as they are easy to find/make and give plenty of relationship points.
For Phyllis: Yellow Sapphire Necklace (+15 relation, craft with 2 Topaz and 2 Wild Cocoon, recipe book from Ginger at 2 hearts) and Talisman (+10 relation, craft with 2 Marble and 2 Wood).
For Gust: Structural Aesthetics, Five Hundred Creative Designs, Structural Dimension Theory (+15 relation, found in Abandoned Ruins) and Umbrella (+10 relation, craft with 4 Spines, 2 Copper Bars, 2 Fiber Cloth)
You can use this site to check which gifts are preferred by each character: https://mytimeatportia.fandom.com/wiki/Gift_preferences

From the moment you are friends with a romanceable character, you can confess your love by gifting them a Heart Knot. This is an item that can be bought at Alice's shop for 160 Gols (depending on the market price). However, there is a chance that the character will reject this confession. The higher your relationship level with the character, the lower the chance they will reject. If they reject, you can increase your relationship even further and try gifting the Heart Knot again. If they don't reject, congratulations: you are now dating! Instead of a play-date, you can now invite them on an actual date. With all that hard work done, the last step is relatively simple.

I would suggest you invite them on a date for the next morning between 10am and 12am. This way you can just wait in in your house the next day for the date to end (in this case 12am) to unlock this achievement, and then reload your save to start the day over so you won't lose any relationship points with that person. If you don't care about this (or you have no heart), you can also continue and live with the consequences (minus 20 relationship points).

I hope this helps. If there are things incorrect or not clear, please leave a comment.