For many campervan conversion related jobs, the first thing you need to do is disconnect the battery.  This is primarily to avoid electrocuting yourself, but also in some cases to prevent potential damage caused while doing the work.

An example of a job which you should disconnect the battery for is fitting a seat swivel - many seats in more modern vans have wiring in them for airbags, seat heating, even electric adjustments.  Other jobs include fitting a split charge relay and any other electric wiring tasks.

Be aware that disconnecting the battery can result in your alarm needing reset and the radio/cd player needing a code to be entered before it will work again.  So make sure you have codes before you disconnect it!

When doing anything with the battery the number one rule is don't accidentally touch both negative and positive terminals at the same time.  If you do you will receive a shock!  Be especially careful when undoing a terminal with a socket wrench that you don't twist it round and hit the other terminal!  You also need to be aware that connecting the positive terminal to anything metal while the negative earth lead is still connected will also result in a shock!

Disconnecting the battery is very straightforward - you need a socket set with the right size socket, open the bonnet, find the battery, take the cover off it if there is one, undo the negative terminal and tuck the earth lead well away from anywhere where it could accidentally touch the positive terminal.  Reverse this procedure to re-connect it, all the time being very careful not to touch both terminals at once.