1
Please wear a facemask inside.
Sign 'please', then sign 'facemask' by mimicking putting a mask over your ears, followed by 'wear' and 'inside'. Make sure to use a polite facial expression to match the request.
2
I forgot my facemask at home.
Point to yourself for 'I', sign 'forget', then sign 'facemask' by bringing your 'C' shaped hands from your mouth to your ears, and finish with 'home'. You can use a slightly frustrated expression.
3
Do you have a spare facemask?
Sign 'you', 'have', 'extra', and then 'facemask' using the motion of securing a mask over your face. Remember to raise your eyebrows and lean forward slightly to indicate a yes/no question.