how do i replace starter or solenoid in 2002 safari?

When I turn the key, the dash lights go dim and voltage drops. Even with heavy duty jumper cables on a known good battery in a running car, the same symptoms appear. Right after Christmas, I would prefer to replace just the solenoid, if that is the problem.
What is the best way to get it started in the meantime? Can I just put 12v directly to the starter?