Make cards for the things that make sense. Estimate them like all the others. Estimating is scary. We'll talk about how to deal with errors shortly. With all the developers together, estimates will tend to average out.
Developers divide up into teams of two or three.
Each team looks at each story. Reflect on how you are going to do it in the system, using your experience with the System Spike Solution to guide you. Talk through an option, estimate how long each step would take. If you think of an easier alternative, or someone says they think they can do it in less time, take the smaller number. When the other teams look at a card that has already been estimated, they can reduce the time but not increase it.