For the assembly, use a M6 bolt and nut and a M4 bolt and nut. You can also use some two layers of 1mm foam sheets on part A and B each.
Remove all supports and make sure that all parts are rigid. Use some grinding paper if the parts don't fit. directly. Insert the bolts and nuts (if the nuts don't fit you can heat them up a bit but be careful not to deform the model) stick the foam sheet on pieces A and B. Attach to stearing wheel.