Puddles are a rare feature on model railway layouts and modelling them depends a bit on the surface of your yard. I took a quick trip out in the rain to take some photos and can suggest a couple of options.
If the puddles are on tarmac then as long as you have a dip in the surface, this can be filled with a suitable water substitute. Gloss varnish would probably do, but I've found Deluxe Materials Solid Water to be really effective. This is a two-part formula that you mix up and drip where the water should be. Both will dry clear, so adding a tiny amount of dark grey or gloss black into the mix might be a good idea for a real oily puddle.
If the ground is rubble then puddles are best added before you lay the surface. Take a bit of clear plastic – the packing from shirt collars is good for this as it's quite thick – and paint one side dark grey. Stick the plastic to your baseboard paint side down and bury it with scatter, plaster, or in this case ash.
Wipe the puddle clean with a damp fingertip and then fix the ground cover in place with diluted PVA. If the water isn't shiny enough, polish the plastic with some Brasso and a cotton bud.
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