how a welding fixture is designed?

Question

the procedure for designing and various steps to be taken while designing

Answer 1

A welding fixture is a jig to hold the parts being welded. The design depends on what you are welding. Finishing the work properly requires operator skill because hot welds can distort the piece and pull it out of shape. A welding fixture can be as simple as some guide pieces tack-welded to the table...whatever you need to get the job done.
Simple fixtures...
http://www.vansantent.com/magnetic_welding_fixtures.htm
Universal workstations...
http://www.bluco.com/welding/systemsizes/index.html
Fixtures for automated welding...
http://www.premiertool.com/id74.htm