Ok, so first off, you know that you are going to need some alginate, some impression trays, gloves, etc.
Did you know that by fluffing or aerating the alginate powder will help you get a more accurate water/powder ratio. Kind of like sifting flours for your favorite chocolate chip cookies. Then if you will add the powder after you have put the water in the bowl, it makes it easier to mix, also like when you are making cookies. Don't forget to tap the scoop with powder and level off the top. This will cause too much powder into the mixture and will make your alginate too thick. Two scoops is generally the norm for a full arch impression, but be sure to always read the manufactures guidelines with each different kind of alginate before you begin.
Use cooler water to make the allow more working time. Hot water tends to set it up a little faster.
When mixing, I bet we all wish we could be using grandma's good old Kitchen Aid mixer. Go ahead and mix powder and water together and be sure to press the mixture against the side of the bowl as you rotate it in order to get all the extra air bubbles pressed out. Once again, after checking the materials guidelines; should be around one minute of mixing, make sure that your alginate mix is a soft creamy fusion that is smooth but also does not fall off of your spatula.
Now the race to get your "cookies" into the oven begins. You need to scoop the alginate from all sides of the bowl, but at the same time, try not to let any air bubbles back in. Then spread evenly in the tray. Set time for most manufacturers is three minutes from the start of mixing. Fast-set is two minutes.
Now time to open the oven and start the baking!