Grab them, break them up, put them in the oven at about 200F for a while and they'll be dry and ready to go. If you want painted ones you can paint them once they're on.
Sorry for hanging on to this matter, but if I understand correctly, you remove the seeds/pods from the catkins before drying them in the oven?
Do you use a tool or just your fingers?
I collected about ˝ liter of catkins yesterday, and I want to try producing some leaves