I have been thinking about buying a pair of polystyrene foot lasts (from here or here for example) to help get good results when making felted slippers. A few years ago I made several hats from this great Chicknits pattern, and know that the robust blocking of the wet, fulled garment is the most important step in getting a happy result. Here's one of my hats.
But the polystyrene lasts are expensive, especially if you aren't making many pairs of slippers the same size. So, I had a quick Google and found this useful page: Make your own shoe lasts for felted boot projects written by Sally Pointer in 2004. Luckily I had some of the necessary silver tape in the house, so I went ahead and made some lasts.
Because I was planning to make slippers not boots, I didn't include the piece of card to give a flat sole shape. This may or may not have been a good move, but once I've worn my slippers for a while I'll know what to do the next time!
I used the lasts as I would have used my own feet - at the drying stage, when the slippers had been completely fulled in the washing machine. I'm happy with the results, and it was certainly easier than wearing wet slippers. With hindsight it might have been a good idea to make the toes rounder, as that part of the slipper wasn't stretched enough by the lasts.
If I find my new slippers comfortable and easy to walk in, I may still invest in a polystyrene set of lasts - I like the idea of using them in the washing machine for the last step of the fulling process. My home made lasts are a little squishy and I'm not confident about their water resistance.