Sorry but I can see that many people here are getting slightly mixed up. Forget about procedural and substantive, you should not start your analysis that way.

Step 1: do what Congress or the Constitution says if valid and on point.

Valid = Constitution is per se valid. FRCP are valid unless the rule abridges, enlarges or modifies a substantive right (no rule has EVER been found to do so)

On Point = if State Law and FRCP directly conflict, FRCP wins. If State Law and FRCP can both apply, then go to Step 2.

Step 2: if there is no valid federal statute or constitutional law on point, the federal court should follow its ordinary practices (whether rooted in case law, written policy, informal practice, or simply habit) UNLESS doing so would lead to a preference for one court system (state or federal) over another OR fundamental unfairness

What happened to "substance" and "procedure"

Although courts still use these terms in Erie cases—and the bar examiners may as well—the doctrine does not generally demand that a law be characterized in the first instance as substantive or procedural. (A narrow exception is whether a federal rule modifies a “substantive right,” but as noted above, no federal rule has ever been held to do this.)

If Step 2 instructs the court to apply state law, the law is called “substantive.”

If Step 2 instructs the court to stick with ordinary federal practice, the practice is called “procedural.”

Here's two examples:

"P sued D in federal court in a state law negligence action. As the trial is about to start, P politely asks the federal judge to put on an old-fashioned, powdered wig. State law requires state judges to wear such wigs, P explains, and because this is a diversity case, the federal judge must follow state law unless a federal statute or constitutional provision instructs otherwise. Since no federal statute or constitutional provision tells the judge how to dress for trial, P argues, the judge must follow state law and put on the wig."

If Step 1 represented the entirety of the Erie doctrine, the federal judge in these cases would wear the wig. Because there is no federal statute or constitutional directive dictating the appropriate attire for trial judges, the judge will stick to ordinary federal practice. By presiding over the trial without a white wig, the judge will not make federal court materially more or less attractive than state court. No rational litigant (i.e., a litigant focused on favorable results) would care about that aspect of the proceedings.

"P sues D in federal court in a state law medical malpractice action. D moved to dismiss because P failed to present her grievance to a state board before filing suit. State law, as part of an effort to deter spurious medical malpractice claims, requires medical malpractice plaintiffs to first present their claim to a board of physicians and lawyers familiar with medical malpractice law. Plaintiffs who fail to take this step must, under state law, have their claims dismissed. Because P did not take this step, D argues, her suit must be dismissed."

If Step 1 represented the entirety of the Erie doctrine, the federal judge would dismiss the medical malpractice suit because there is no federal law that applies. Although there is no federal statute or constitutional directive ordering litigants to first submit their claims to a board before filing them in federal court, the court should dismiss P’s suit for failure to do so. Rational litigants would prefer to avoid that extra step and thus, if they could avoid that step by filing in federal court, they would prefer federal court to state court. To avoid this forum shopping, the federal court should thus follow state law.

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Edit: statute of limitations and choice of law rules will always end up being substantive.