You lose Mana equal to the total Mana Tax of all of your installed Cyberware. If you would have more Mana Tax than Mana, then you have 0 Mana. You can't go into negatives, however, for every Mana Tax that isn't paid upon installation, gain a Scar.
Example: Jim-Bob Jones wishes to install a Mk. 10 Reinforced Synth-Skin, which has a Mana Tax of 3 but he only has a total of 1 Mana. Therefore, if he were to install it, he would have 0 Mana and immediately gain 2 Scars.