First of all, you have to open your internet browser. Make sure that your internet connection is working. Go to the website Code.google.com and search for 'gappsinstaller.apk' file. When you find the file, tap on it and download it to your tablet.
Usually, the file will run itself after downloading. However, if it doesn't run itself, just open it to install it on your tablet. The method of installation differs and depends upon the model of your Android tablet. If you are using Archos 101, just open the 'Files' app from your device and tap the APK file icon. On the other hand, if you are using some other model, you can contact with your tablet’s manufacturer to get the particular instructions to install the app.
Now, tap 'Install Core Apps' when indicated on your screen. It will install Google Apps and Android market on your device. After the installation is completed, just reboot your tablet in order to finalise the process.