The two main steps to become a longshoreman are to get a dockworker's card and then work your way up through the local union. In the United States and Canada, longshoremen belong to organizations such as the International Longshoremen's Association (ILA) and the International Longshore and Warehouse Union (ILWU). After you have obtained the necessary credentials to work on a dock, you can start working as a longshoreman. You will typically begin as an "unidentified casual," which means that you will be the last one to receive work on any given day.
If you want to become a longshoreman, the first step is to obtain the necessary credentials to work on a dock. This can be referred to as a dockworker's card, though in the United States the technical term is Transportation Worker Identification Credential (TWIC™). The TWIC™ is a common credential that is used by anyone who needs to have access to secure Marine Security Transportation Act (MTSA) areas, such as those found in ports. To obtain this credential, you will need to provide documents that prove your identity and have both your fingerprints and picture taken.
After you have obtained your TWIC™, you may look for work as a longshoreman. The first step in this process is typically to contact your local chapter of the ILWU, ILA, or other applicable union. They will be able to provide you with the specific procedure for "unidentified casuals" as well as any other specific information that you may need to know. They may require that you go through an application process and pass various written or practical tests. Since it can be very difficult to get shifts at that stage, you may want to look for temporary work in construction or another related field while you are trying to become a longshoreman.
Whether or not you get any work as an "unidentified casual" can depend on your location, the economy, and a variety of different factors. If there is a lot of shipping in your area, there is a better chance of there being enough work to go around. After you have logged enough shifts at that stage, you may be invited to become an "identified casual." This is the next step to become a longshoreman, since you will be somewhat more likely to obtain work each day.
The next step to become a longshoreman comes after you have worked a sufficient number of hours as an "identified casual." At that point you might be invited to join the ILWU, though you will not be a dues paying member of the local chapter. You are much more likely to be offered a shift on any given day as a "class B" member, though it is not until you become a "class A" member of the local union that you are more or less assured to get work when it is available.