Get the alginate from a dental supplier. I can't remember the trade name but ours is bound to be different to yours. The dental stuff is fast setting. It's got this colour substance in it. it goes purple ( actually violet
) to begin with while you mix it. When it goes pink, you load it into the tray. I just used a cut down celestial seasonings tea box lined with tin foil. Then when it goes white, you put the baby's foot in there & hold it there for about a minute until it sets. I find getting someone else to hold the baby & giving her something to play with to distract her from wriggling too much really worked well. You can do hands too like that, but it has to be something really interesting they're not normally allowed to have. You then get this solid jelly type mould that you pour the plaster of paris into. The alginate stuff I got remained structurally sound for 100 hrs but I was not prepared to risk that since it is really humid here at the mo' so I pretty much poured it straight away. When the foot's set, you just gently peel off the mould. It gives the most beautiful feet, all the little lines & the skin texture is there. I love using it. Just those darn bubbles ! I guess you could fill the bubbles if there were only a few. One other thing I found out too late, make sure there's no dirt of the baby's feet or that'll end up in there too!