Hi Victor,
Its correct that the previous ones will move around a bit. So, what you try to do is get back to the previous ones and try to twist them again. Just keep on twisting and moving the whole unit as you twist :). Hope to help

I made this model years ago and only once so I don’t quite remember if I had same problems with the assembly.

Is it a good idea to hold the curls with hairpins while assembling the model?

If you feel like you need it then yes, sure! Sometimes I use this kind of pins for tricky models, although I never had to use them with curlers.
You might also want to use a wooden skewer (or a toothpick) to connect the curls together. At least that’s how I do it.
Anyway, it’s still plenty of time till the end of May, I hope your grandmother gets a great kusudama for her birthday, if not this one then maybe something else!:)

Yes, I think I was able to do the curls properly. Each flower stands firm individually and I can assemble up to five flowers without trouble. My problems begin when I try to add the sixth flower. In fact, it seems to me that, for every new flower, the curls that connect the previously assembled units tend to move a little bit and need to be reinforced. Am I correct? is it normal? Since the paper I’m using is slightly rigid (120g/m²), the problem may be that it lacks sufficient flexibility to adapt and make room for new units. With recycled paper (75g/m²), I didn’t have that much trouble. Is it a good idea to hold the curls with hairpins while assembling the model? This is the paper I’m using, but in two different colours (golden white and pearly pink): . Sorry I couldn’t find more details about it in English.

I thank you deeply for your time, attention, patience and help. Hopefully, I’ll be able to finish this origami before the end of May, for my grandmother’s birthday. =)

Is the paper too thick?

Hm, it might be… Are you able to curl the flaps properly?
The thing is that the weight of the paper is not the only factor that matters here. The most important quality for such model is the “memory” of the paper, or its ability to remember a fold (a curl in our case). So some weak or soft papers (thin or thick) will not work with curlers at all.

I’ve been trying to fold this model with a relatively thick paper (120g/m²). Each flower looks amazing, but I’m having lots of trouble trying to assemble the kusudama. Is the paper too thick?

There are 12 flowers here, each made from 5 units (60 units in total).

I’m not sure there is a diagram for this model available online somewhere… But you can contact the designer directly and ask, here is her website: http://www.origami.edu.pl/