Hi Smudger - Welcome to the forum. There are an increasing number of us on here from the UK and it is good that you have joined us.
You have made a good choice with the WSM. I assume when you said it was mid size that you bought the 47cm? This is a good all-rounder and should be good to feed family and friends. The first thing you will need to master is the fine temperature control but with a WSM this is straightforward. The next important thing is the choice of fuel. Charcoal a most briquettes are designed for short duration and high heat for grilling so for the Low and Slow cooking you need a harder briquette that will give more sustained heat over a longer period of time. One I would recommend for smoking is Heat Beads. They are more expensive to buy but 1.5 - 2 Kg of these will give you 7-8 hours of cooking. Some use all wood on their WSM but personally I would not recommend it.
For long cooking you need to use the Minion method for the coals
The photos below are actually with a ProQ Frontier but the preparation and control is exactly the same as with the WSM.
Place a can of beans or equivalent in the centre of the fire basket and pour the briquettes around it. Pull out the can to expose a hole in the centre. This is where you are going to put your burning coals.
Get about a dozen briquettes up to temperature in the chimney starter and pour these into the centre
You can then lay your wood of choice on top of the unlit briquettes with some just touching the lit briquettes. Above you can see an example of wood chunk and pellets on top. These will burn slowly over the duration of the cook providing continuous light smoke. 3 or 4 good handfuls of chips or pellets will be fine over the coals. There is no need to soak the chips.
Open the bottom vents fully.
Half fill the water pan with either water or sand and put it in place. The main purpose of this is to act as a heat buffer between the coals and the cooking chamber. As this is your first time then I would recommend that you use water.
Attach the body of the WSM. What kind of thermometer do you have? Is it designed to have a probe permanently inside the cooking chamber? (e.g Maverick ET-732/733). If so then clip the probe of your digital thermometer onto the cooking grate. If you do not have a thermometer with a probe designed to stay in the cooking chamber then you may want to get one as the lid thermometers are usually quite inaccurate.
Place the lid on the WSM with the top vents open and monitor the temperature as it rises. This will take 20-40 minutes. As the temperature gets to about 3/4 of the desired smoking temperature begin to close the bottom vents in stages. You need to find the balance where there is enough air going over the coals to produce enough heat to just keep the cooking chamber at the desired temperature. Do not let it over run as it is much easier to increase the temperature than it is to reduce it. Do not close the bottom vents completely or the coals will go out.
With regards to recipes, what are you looking to cook? Pork shoulder and/or ribs are a good start as they are quite forgiving. Let us know and we will put up some recipes for you.