Hello to both!
I hope the following may give something of a guide regarding Wet in Wet Watercolour;
The first thing to be aware of is the quality / weight of the Watercolour Paper; if you wish to have a good go at wet in wet, then the heavier the paper, the better. 140lb (300gsm) should be the absolute lightest paper to consider until more experience is gained. So try to use the heaviest paper you can afford, though most manufacturers produce 200lb paper, and this is OK.
If you're using a heavier paper, or a Watercolour Paper Block, you can get away without stretching the paper, but if you're using a lighter paper, you'll need to stretch it by soaking the paper in a bath, or large bowl for a minute or so, then tape it down onto a board surface. When the paper is dry and stretched, it is much less likely to cockle, or ripple when wetted again.
Wetting the paper can be done in basically a couple of ways; either with a Large Flat, say around 1", or something like a No.16 Round if you don't have a Flat, or alternatively, as some professionals use, a natural sponge. Some professionals use a sponge as they say they can control the water better, but this is a personal choice, so try both and see which suits you best.
After applying the (clean!) water to the paper in your preferred way, as evenly as possible, allow the paper to partly dry off so that instead of being wet and shiny, the paper just has a slightly damp sheen to it, and this is the best time to drop in wet colours.
I hope this helps, but if you need more information, then come back to me.
Very best wishes,
RSC Wigan / Warrington area