Installing memory modules is straightforward. Most recent motherboards automatically detect installed memory modules regardless of the slot they occupy, but it is good practice to install modules in the lowest numbered slots first. For example, if a single-channel memory motherboard has four memory slots, they will be numbered 0 to 3 (or 1 to 4). Fill slot 0 (or 1) first, then the other slots sequentially as you add modules. If you are installing memory in a dual-channel memory motherboard, install memory modules in pairs, filling the lowest numbered slots first. For example, if the motherboard has two slots each for channel A and channel B, numbered 0 and 1, fill the slots for channel A slot 0 and channel B slot 0 first.
Some motherboards require higher-capacity modules to be installed in lower-numbered slots. For example, if you are installing two 256 MB DIMMs in a dual-channel motherboard that has four DIMM sockets, with 128 MB DIMMs already installed in the 0 slots for channel A and channel B, you may have to move those 128 MB DIMMs to the 1 slots for channel A and channel B and install the new 256 MB DIMMs in the 0 slots for both channels.
That rule is not invariable, though. A few motherboards require smaller modules to be installed in the lower banks. Some motherboards don't care which module you install in which bank. Best practice is to check the manual before installing memory. If no documentation is available, experiment by moving modules around. If some ...