1. Download Lineage 2 Interlude you can find it for example download button here or use Google to find L2 Interlude Client. 
2. Install the game, and then you download our System folder.
3. Copy our System to your Lineage 2 folder.
4. To play with Certus you have to load the game from this path C:\Program Files\Lineage2\System\L2.exe of course the whole team wishes you great time with L2 Certus.