How much sand or gravel do I need to use?

Yes, you may use decaying wood dirt. In the wild, pill bugs feed on decaying wood.

Gravel has no expiration date, unless the manufacturer specifically states this. It is the quality of water that determines what should be done with the substrate in a system. As...

You could buy snails or shrimp, which will eat the algae.

Yep! I use a mesh colander to clean gravel. You shouldn't be cleaning gravel after it's added to a tank, however.

Yes, the article did say you could dry the sand overnight. There were 2 options: to bake the sand and to dry it overnight naturally.

Yes, this is a good idea. Particle board is a pretty rough material, so take some time to sand any rough edges before you get started.

Yes, 6-mil minimum. Concrete is not only porous (it's a sponge), but it's colder than the ground because its thermal retention isn't as good as dirt, clay, sand or what...

It can depend on the type of soil you have. For sandy soil, 2" of gravel would be good. But on clay or loam soil, that doesn't drain well, a...

It's up to you, but I think it would be easier to just empty the tank, rinse the gravel, and re-fill it rather than change the water daily.

I recommend sanding down the wood for a smooth finish. After, apply primer, sanding in between coats of primer in order to get a better adhesion to the paint.