I don't understand how the first part of the "flowerflow" works. I know it begins by a text storage but the other inputs are strange... Could somebody explain me that ?

As you said, you first do a text storage on the flower. Don't mash the text, you need to do the next inputs before the text box finishes. After that, you open the menu and press Z twice so the cursor is on "USE" for the punch card.

When the cursor is on USE, do these inputs quickly: Enter - Shift - Z This opens the punch card and closes the text box.

If you have further questions, you can join the Undertale speedrunning Discord server and ask the members there :) https://discord.gg/Gbz8YTj

thanks