How should I start with this?

First you need to set the desired voltage and current limit. To charge a 12-volt lead acid battery, which has 6 cells in series, you need to set a voltage limit of 2.15-2.4V per each cell. So if it has got six of these in series you need to set the charging voltage limit to 12.9-14.4V.

The recommended charging current is between 10 and 30 percent of the rated capacity. So a 9Ah battery should be charged at about 2.5A, or lower. Once the battery is fully charged the current drops to a few percent of the rated Ah, which indicates the battery is charged.

how much do I loose if I try to charge it from 12v?

Anything above 2.15V/cell will charge a lead acid battery. This means below 2.15V/cell no charging will occur.

https://electronics.stackexchange.com/questions/336759/charging-12v-9ah-lead-acid-battery