How to use Emdebian Grip on your own systems
Ensure your device is supportable
The device needs to be one of the architectures supported by Grip: armel, mips, mipsel, powerpc, i386, amd64 and SH4 (unstable only). Old style ARM devices are not supported. Check for arm-linux-gnueabi support or EABI.
Emdebian Grip will avoid the need to cross-compile the operating system itself.
What about packages of my own?
You'll still have to compile your own application unless it is in a supported interpreted language. To add a package that is in Debian, you would use apt-grip (part of the emdebian-grip package) to download the binary package from Debian and process that binary for Grip. It's then a question of getting that binary onto the device along with any other dependencies that your application might need. The limitation with apt-grip is that it needs to be run on a machine of the same architecture as the device.
The most flexible way to have your own packages is to maintain your own mini-Grip repository which you can keep automatically updated direct from Debian using the scripts in the emdebian-grip-server package. The process is mostly automated but you will need to know about reprepro and mirror management to get it working smoothly. This method is architecture neutral.
Once you have a local repository, you can combine that repository with the standard Emdebian Grip repositories to create a root filesystem which gets installed onto the device. Use multistrap for that purpose.
The details depend on just what kind of methods you have available for installing stuff onto the device, how much storage it has, what sort of other devices are attached (like keyboards or monitors to allow you to work on the device or whether you have to work on a desktop and copy to the device) etc.
You will also need your own kernel, unless you can use one of the few standard Debian armel kernels.