Introduction

(1) When to start?
Start as early as possible, may be 12 months before your planned date, but you can get spots even by applying 2-3 months before.

(2) Things to gather before Applying?

Transcripts from your college/university(a sample can be downloaded from the bottom of the webpage)

Immunization (One can get it signed from any doctor, GP or a professor, doesn’t matter)

Malpractice insurance for 1 year

Health Insurance

HIPAA compliance training (Few universities require that like John Hopkins and U of Vermont etc)

Transcript is a set of documents given from the college as an assessment of all you have done. It contains all your postings(ward rotations/ clerkships) details.. marks.. etc.. there is not fixed format for it.. Each college has its own format. It is not usually given before completion of your degree. In short, you need a documentation of the clerkships you have done during mbbs and the details of your marks of all your years with a official seal/ stamp on the documents. Thanks to vaibhav for this valuable description!
A sample can be downloaded from the bottom of this webpage, thanks to medisid!

That’s pretty much all the stuff that is required at every university, then get your application signed from Registrar and send the package.

(3) Places to apply?
Go to this link if you want to prepare a list for your yourself otherwise I have also created a list for IMGs with direct links to the IMGs electives page in the university's website.
There is a list of places offering electives in the first link, easiest way is to search schools by geographic region, select “Schools that accept international students for electives” and select any region to search medical schools that accept international visiting students. Just confirm if a medical school requires Step 1 as well or not.
But there are many places that provide electives but are not listed over here, moreover sometimes the information here is not updated. Anyways it’s a good place to get an idea of the universities where you would like to go.

(4) Acceptance?

APPLY EARLY and try to maintain good relations from Electives coordinator because he/she will be the person who will deal with your application altogether.