Here are things we can do to make the process easier:
Leave the cat alone
Stray cats are usually very independent. Watching a stray cat using a litter box is intruding her privacy. Place the litter box in a private, quiet, and accessible location and let them take care of their own business. Their instincts will them to use it if they feel like urinating.
Make it natural
Outdoor, stray cats are used to natural soil and sand and hardly ever used to cat litter. Because of this, the litter may seem alien to the cat. To make the difference less prominent, design their litter boxes to mimic the natural environment. Put some rocks, pebbles, and dry leaves to encourage them to use the litter box a place to bury their waste. This environment will feel more comfortable to a stray cat.
Give the cat some reward
Forcing the cats to use the litter box is not the right way of training. It may even scare the cats, making those felines hate the boxes (and you). They will learn by watching your behavior. After feeding them, stir the litter. Do it regularly and they will link these two activities together. They will understand that the litter box is the place to go when they feel full.
Each time the cats succeed using the box, give them some treats. Buy some wet foods or catnips to appreciate the achievement. If they fail, don’t punish them. Cats do not understand punishment. They will think you are just being mean.