Fit your life jacket on shore, over the clothes you plan to paddle in. Check the label, fasten it fully and adjust it until it is comfortably snug. A life jacket stored in the boat cannot do the same job as an appropriate one you are already wearing.
For a beginner paddling outing, the simplest starting point is equipment recommended by the instructor or outfitter for your activity, size and conditions. Do not choose only by color or assume that an adult-size jacket will fit every adult equally well.
Read the approval and use information
In the United States, look for the appropriate U.S. Coast Guard approval and follow the label's size and activity information. Other countries have their own standards and local requirements. Check the rules where you are actually going rather than assuming an online description is the law everywhere.
The Coast Guard's life-jacket guidance explains that designs differ and must be worn correctly. Inflatable and foam-filled products are not interchangeable for every user or activity. If you do not understand the operating instructions, ask before taking it onto the water.
Try the fit with your paddling clothes
Put on the jacket over the layers you expect to wear. Fasten every required buckle or zipper and adjust the straps according to its instructions. It should feel secure without making ordinary breathing or arm movement uncomfortable.
Minnesota's Department of Natural Resources recommends trying a life jacket with your paddling clothes and checking that it allows free movement. Reach forward as you would to hold a paddle. If the jacket pushes into your chin or prevents comfortable movement, ask for another fit rather than leaving fastenings undone.
Following the fit-check instructions, gently check for upward movement at the shoulders. The jacket should not ride above your chin or ears. An instructor can help you distinguish a strap that needs adjustment from a jacket that is the wrong size or shape.
Check condition as well as size
Look for damaged fastenings, torn fabric or other deterioration before use. Follow the manufacturer's inspection instructions, particularly for inflatable equipment with service requirements. Tell the rental staff about a problem and request a suitable replacement; do not improvise repairs at the launch point.
Children need a properly selected child life jacket, not a small adult jacket tightened as far as it will go. Keep their fitting and supervision requirements separate from an adult's. The National Park Service emphasizes choosing the right jacket for the activity and wearer.
Wear it as part of the outing
Make the final check before boarding, when both hands are free and help is nearby. Keep it on and properly fastened while paddling. If a layer changes, reassess the fit rather than assuming the morning adjustment still works.
A life jacket supports a water-safety plan; it does not remove the need for suitable conditions, instruction and supervision. Start with an outing appropriate to your experience. A properly fitted jacket should be ordinary preparation, not something you first try to understand after falling into the water.