Hi & welcome, @Clara
Here is the link to that post that @MrDoo posted: Kristina Shin Basic underwire bra tutorial - #8 by MrDoo. I haven’t spent enough time to test it.
Another option is to use the program Ghost-it. Again, I haven’t tested it. I believe it only works on Windows and that the project has been abandoned, so there will be no further updates.
No, Seamly works a bit different from other graphic software in that each point fans out from a start point and rely on each point that went before.If the sequence is broken, then the pattern will crash.
If you could map out the pattern like on this website, then you could draw your pattern very easily in Seamly without using a background image. I’ve created a few short videos creating this pattern in Seamly: https://www.youtube.com/watch?v=KW6l3khH0dk
I hope this helps you 