Short answer
Press-on nails are pre-shaped cosmetic nail pieces selected by width, curve, length, finish and attachment method. They are not a universal fit or a guaranteed wear period. Read all kit directions and warnings before use, including preparation, adhesive, removal and storage instructions.
Check fit before attachment
Lay out sizes for each nail without adhesive. Compare side-to-side width and surface curve. A piece that presses into surrounding skin or leaves a large unsupported gap is not a useful fit. Do not force a larger or more curved piece flat. Keep each selected size in order so adhesive steps can be followed without rushing.
Length is a task decision. Typing, fastening clothing, opening containers and other daily actions can make one length more or less practical, but shorter is not automatically safer or better. If the kit permits shaping before application, use a clean appropriate file and follow its directions. Do not cut or aggressively file living skin or the natural nail.
Apply and remove by the label
Use only the supplied or specifically directed attachment method. More adhesive does not prove longer wear. Keep products away from eyes and follow ventilation or flammability warnings. Remove the set according to the manufacturer rather than pulling pieces away by force. Stop if the nail area becomes painful or otherwise concerning.
Common questions
Should a press-on touch both sidewalls tightly? It should fit according to the kit directions without pressing painfully into surrounding skin. Do not force a mismatched curve.
Can every press-on set be filed after application? Check the specific instructions; material and finish differ between products.
Sources: FDA nail-care product information and American Academy of Dermatology nail-trimming guidance.