What are you starting with? A NEURON program that contains a model cell, or a file that contains morphometric data (like a Neurolucida or .swc file?). If the former, use the program to set up the model cell, then bring up a CellBuilder
NEURON Main Menu / Build / Cell Builder
1. click on the CellBuilder's Management radio button
2. click on the Import radio button
3. click on the rectangular Import button
and the CellBuilder will now contain the anatomical specification of the model cell (its branched topology and the detailed diameters and lengths of its various neurites).
4. save the CellBuilder to a ses file for future reference--you might want to use it to insert biophysical mechanisms or to specify a spatial discretization strategy
5. click on the Export radio button
6. click on the rectangular "Export to file" button
and tell the CellBuilder the name of the file you want it to write, and where to write it.
If you're starting with a morphometric data file, use the Import3D tool to get the morphology into a CellBuilder (see the Import3D tutorial on the Documentation page http://www.neuron.yale.edu/neuron/docs
), then follow steps 4-6.