Consult with Republicans and Democrats. Get to know each candidates' views on major policies, including President Bush's positions. Scrub their public records.
And keep it quiet.
Three months later, the Democratic presidential candidate and a tiny circle of advisers have kept faith with those goals in a secretive, methodical and wide-ranging search.
But the hardest days are ahead for Kerry as he comes to the end of a process designed to produce a topflight running mate and a positive first impression of his decision-making skills.
"This is a defining moment for any presidential candidate," said Democratic consultant Ron Klain, who worked for former Vice President Al Gore (search) in the White House and on his 2000 campaign. "How you go about picking the person says as much about you as who you eventually choose."
Keenly aware of the impact, Kerry goes out of his way to explain that he is showing "great respect" for the process by keeping it secret, thus dignified. His advisers refuse to talk about potential candidates, but they cough up details that cast their boss as a steady and strategic decision-maker.
In addition to the ground rules he set for the process, Kerry has given Johnson a "mission statement" that outlines what he is looking for in a running mate, starting with somebody who would not miss a step as president should something happen to Kerry.
Kerry has studied vice presidential searches since the 1930s, pledging not to repeat the mistakes of past nominees.
To promote his decision-making skills, advisers point to crossroads in Kerry's political past.
Last year, the four-term Massachusetts senator chose a high-risk strategy and gambled his candidacy on the Iowa's caucuses when he was foundering in neighboring New Hampshire. In the 1990s, he undertook an uphill campaign to normalize relations with Vietnam.
That steady-as-he-goes portrait does not always jibe with recollection of those who served with Kerry before the presidential campaign. They say he agonizes over decisions, reaches out beyond the structure of his staff for advice and waits until the last minute to make up his mind.
They compare his style with former President Clinton's. The Arkansas Democrat was a famously "circular" thinker who bucked his staff's attempts to place deadlines and limits on his decision-making. Former aides say Kerry's instincts for policy decisions are stronger than for political questions such as the one before him now.
Those who have worked with Kerry, in this campaign and others, seem to agree that while his thought process may be as roundabout as Clinton's, it has a purpose. Like the prosecutor he once was, Kerry relentlessly collects facts and opinions, mulls over the evidence and unveils his case on his own timetable, they say.
The senator has allowed only two advisers inside his vice presidential deliberations — Johnson and campaign manager Mary Beth Cahill. Most times, neither of them knows which candidate Kerry might favor.
"He keeps this very close to his chest," Cahill said.
"He spends a lot of time marshaling the facts. He views this as a very important decision, and he has invested a huge amount of time in reading, meeting and thinking about this," she said.
Since early March, Kerry has met with Cahill and Johnson at least once out of every 10 days to discuss the search.
Johnson and his team of advisers have thoroughly checked the backgrounds of Rep. Dick Gephardt, D-Mo., Sen. John Edwards, D-N.C. and Iowa Gov. Tom Vilsack. Kerry has met privately with Gephardt and Sen. Bob Graham, D-Fla., to discuss the vice presidency.
Beyond that, little is known for sure — and Democrats believe there are other candidates under consideration. They note that vice presidential nominations tend to go to people whose names were not widely circulated as candidates.
Kerry must make his choice before the conclusion of the Democratic National Convention, which begins July 26. Aides hope he will settle on a nominee several days before the convention; some suspect it will be a last-minute decision.
Despite the paucity of leaks, Kerry has chastised the media and his staff of churning the rumor mill.
Addressing a large crowd at a Women for Kerry luncheon fund-raiser on Friday, he said: "I'm standing here thinking that I'm going to read a headline in the paper tomorrow — Kerry Busy Interviewing 2,500 Candidates for the Vice President of the United States."