yep - it is really that easy.
do you have fish in there??
OK, what you need to do is use your siphon to empty about 50% of your water (turn off all your filters and heaters while you do this)
now, drop in as much prime (or whatever dechlorinator you are using) for the amount of your tank. so if you have a 20 gallon tank, dose for 20 gallons.
now hook up your siphon to you your sink (or in your bucket if you are using buckets) and temp match your water to your tank. you can use your hand to match. once they are matched add your water.
make sense?