Download the latest nymea image for the Raspberry Pi 2/3.
There are different ways how to flash the SD Card. The easiest one is probably to use BalenaEtcher. If you can't or don't want to install a third party tool, you can find instructions for manual flashing at the official Raspberry Pi documentation or here in the wiki.
Now, if you are using a Raspberry Pi with touch screen, this isn't required as nymea:app will be preinstalled on the Raspberry Pi and show up if there's a display attached. But it is still recommended. You will be able to control your nymea instance from all sorts of devices, your mobile phone, your laptop and so on.
Inset the micro SD-Card into your Raspberry Pi 3 and power on the device. Booting might take a few minutes.
As long as the Raspberry Pi does not have any network connection (such as any LAN cable plugged in, or preconfigured WiFi network), nymea starts a Bluetooth service that helps you to get your Pi connected with your Wi-Fi network.
Now that your Raspberry Pi is connected to your Wi-Fi, you are able to connect to nymea. Make sure that the device running nymea:app is in the same network as your Raspberry Pi. This is necessary only for the first time you connect to nymea via the nymea:app. Once you are there, you can connect to nymea:cloud, which will take care of future connections between nymea and nymea:app.
Setting up a cloud connection is really easy. Just tap the navigation in the top right corner, go to "Box Settings" → "Cloud" and enable cloud. After that you need to go to "App Settings" → Cloud Login and register for our free cloud service. Once that's done you are all set. You can now control your devices without being in the same WiFi as your nymea powered Raspberry Pi. You can also now receive push notifications based on your preferences.
That's it! You are ready to add your devices and create some magic for your environment!