Most states require you to "apprentice" under a licensed PE. For example, for Texas:
Experience
Basic Requirements
You must meet the following experience requirements prior to application with the Board:
1. With an accredited engineering degree you must have a minimum of 4 years of active practice in engineering work, of a character satisfactory to the Board, indicating that you are competent to be placed in responsible charge of such work.
2. With a non-accredited degree you must have a minimum of 8 years of the same type of work required of those candidates with an accredited engineering degree.
Types of Experience
During your first few years of experience after graduation, it is imperative that you place an emphasis on gaining experience that is acceptable for licensing purposes. Failure to become licensed can severely limit your potential professional growth.
This experience must demonstrate a clear use of your engineering knowledge, engineering education, and engineering judgment to perform the task, be progressive, of an increasing standard of quality and responsibility in one dominant discipline. Although it is recommended that the engineering experience be obtained while working under the supervision of a licensed professional engineer, this is not a requirement for licensure.
Experience that is considered most acceptable for licensure purposes generally falls into one of two categories: design or analysis.
The most common type of acceptable experience is design. The common denominator in all design projects is the selection and use of recognized engineering principles and methodology to determine a solution to a problem. The final result of design work will almost always be details, plans, or specifications for use in creating a finished product.
The second common type of acceptable experience is analysis. Common features of analysis activities include the use of mathematical modeling and acceptable data collection techniques to assess a problem, and the act of making a learned recommendation based on analytical findings. An analysis activity will almost always result in a conclusive report or recommendation.
It should be noted here that many other types of activities would also be acceptable if your participation in those activities can be described in terms of design or analysis. By asking yourself if you can describe your activity in terms such as "I calculated..., I designed..., I analyzed..., I recommended...," you can almost assure yourself that you are describing design and analysis.
Documentation
While gaining experience, it is equally critical that you document it in such a way that you can summarize it for the Board. As you go about your weekly tasks, you should keep a detailed diary of your activities: the starting and ending dates of the project(s) on which you worked, name and address of each employer, job title(s), the name, present addresses and phone numbers of the engineers and other persons with which you personally worked who can serve as a reference to substantiate your experience, identification of the project, the scope of the project, and the engineering activities that you personally performed.
This information will be transferred from your diary into a summary called the Supplementary Experience Record (SER) and will be submitted with your application for licensure. It should range from about 6 to 12 pages and adequately describe the engineering activities that you have performed for your entire engineering career, starting with the first engagement after graduation from college.
The SER is to be written in the first person describing in active engineering verbs the engineering work that you specifically designed, calculated, evaluated, analyzed, etc.
You are required to submit at least three (3) confidential reference statements which must be from currently licensed professional engineers who are familiar with the work you describe in your SER. (If you are applying for an original license and requesting an examination waiver you must submit five (5) references from currently licensed professional engineers [P.E.]). You select the references and they will verify the quality and scope of the work that you describe in your SER and will provide the Board with an assessment of your character and suitability for licensure on the Reference Statement Form. A professional engineer who is familiar with your work and not your supervisor will be acceptable as a reference. Professional engineers who have not worked with you may review and judge your documented experience and serve as a licensed engineer reference. Such review will be documented on the Reference Statement Form.