STEP 1

Create an account at www.rebrickable.com and add the sets that you own in your profile.

STEP 2

Search the alternate build you wish to build. 

E.g: Search LEGO® DC Batman Batmobile Tumbler (76240) within Rebrickable's website and click 'Build This Set'

STEP 3

On the Build Results page, you can "Change Build Options" and then "Rerun build with new options" (This allows you to do things like ignore color and use items that are similar but not exact, etc.)

STEP 4

If you don't have all the parts needed to complete the build, you can always use your resourcefulness to substitute with other parts! But Rebrickable also lists down where you can buy the parts that you lack.