You lucky thing! If you do an internet search for 'planting up chimney pots', lots of useful info will appear. :)

All sorts of possibilities - if they're tall ones, I use these in borders and beds. Pick a spot, stand the pot in place, fill up with soil (can be garden soil, doesn't matter, its open at the bottom) and then plant something in it - I usually use something like a Phormium or a cascading grass, or a trailing flowering plant, but whatever you fancy looking at that's higher than the rest of the plants in the border.

Otherwise, to use as containers, you either need to find something to use as a base to stop soil falling out at the bottom, yet still allow some drainage, fill it up with potting compost and plant into the top, or find a permanent position on paving for it, without a bottom, which means you'll never be able to move it because the soil will all fall out. To save on potting compost, I usually break up the 'boxes' that summer bedding comes in, made out of polystyrene, and drop that in the bottom third first, then put the compost on top, tamping it down to make sure there's no air gaps at the bottom.

Alternatively, site it somewhere and find a pot that just sits in the top of the chimney pot comfortably, without falling through, and plant into that. The drawback with the latter arrangement is there's a risk the whole thing will blow over in a very windy spot if the plant in the top gets very bushy, but it depends what the chimney pot's made of and its shape - heavy pottery and flared at the base makes this problem less likely.

If, though, the pots are metal, and you want to plant straight into it, it is best to line the inside with something warm, like that soft polystyrene 'fabric' that gets used in packaging, to help insulate the roots of any plant growing within from both heat and cold.