Bambu recommends supplied PLA and a preloaded file for the first P1S test. That is useful because the model and machine settings are already paired.
Load filament through the rear path or AMS. Heat and extrude through the printer menu until material leaves the nozzle cleanly, as shown in the official quick-start guide. Remove the cooled strand after the toolhead parks.
Clean the plate if you touched its print area, install it squarely and select a short internal model. Watch the first layer through the door. It should form a connected surface without corners lifting. Cancel if the nozzle drags plastic, the plate is misaligned or a loose purge strand is being carried into the print.
PLA can soften in a very warm closed chamber. For a short first test, use the plate and door guidance attached to the current Bambu filament profile. On longer PLA jobs, follow Bambu’s ventilation recommendation for the installed plate and room rather than assuming the enclosure must always stay sealed.
Wait for the plate to cool before flexing it. Inspect the base, vertical walls and top. A small seam is normal; a repeating layer shift or scrape is not. Record the filament preset and successful plate type so the first custom project begins from a known result.
If the P1S is still being unpacked or the AMS is not yet connected, back up to the P1S setup guide before running this first print; the general first-print checklist covers habits that apply to any new machine. Once a few models finish cleanly, the P1S calibration routines are the next step for tightening tolerances.