The beauty of lead-acid and SLA batteries, is that they are not too fussy about how you charge them.
The best option is to consult the manafacturer - they will recommend a charging method.
Alternatively, a lot of people recommend charging at a C/5 rate (C = capacity in Ah). So for a 5 Ah battery, charge at 1A. Another common charging rate is C/20, or C/100 (250mA and 50mA, respectively).
Some manafacturers may specify a constant voltage charge. However, I would only use this if the battery is not severely depleted. A REALLY flat battery may be damaged by too much current.