Connecting batteries in a series means connecting the positive terminal of the first battery to the negative terminal of the second battery, and so on for the length of the string. This increases voltage, and batteries must be connected with cables of equal length and resistance to ensure equalization of the load. All batteries in the string will receive the same amount of charge current, though individual battery voltages may vary.
We do not recommend connecting different SLA battery brands, capacities, and ages in a series. This can lead to over-  or undercharging and cause premature aging of the batteries.