Okay, I can't get a world map of runescape so I'll try to walk you through it manually but it would help if you open another window of it instead. First of all you might want to bring 5 or 6 salmon , trout ,fish ,chicken etc. for health but not because of what you will be training on, because of the random events that happen MUCH MORE often than usual.
First of all you need to be a member to use this most effective way. Go to Falador and go to the exit nearest the mace store and general store. Then go west along the path that leads to a big stone wall and a gate.
Go into Taverly and go through white wolf mountain. once you on the other side in catherby, make your way along the path to seers village. Then walk up the road towards Sinclair's mansion and turn west towards that place relishes or something. Then go up to the place where it is gray on the map (in the game it'll be a gray beach with lot's of rocks) then examine the rocks until you see one with yellow letters spelling rocks and walk near it. A crab will come out with 65 health and a level of 13 for its combat, the reason these are so good to train on ( for range , magic , or attack ) is not because they have high levels, but because they hit so little, like a 1 or 2 every 5 minutes or so, this means you'll probably get hit with a 1 or 2 every 7 crabs you kill. This lets your skill hit it's maximum hit point (mith arrows = 7 damage highest each time) many times on one crab! This means you'll get more experience as you get more exp every time you hit a higher damage. Also they drop items like iron picks and iron and coal.
Easily enough if you are training magic then perhaps you can just teleport to Camelot if you have 45 magic.
Non member
There are 2 places to go if you are a non member. If you're a high level just go to karajma where the lesser demons are and attack them ( go 2 the volcano and go through the top) bring food as these demons hit hard!
Low level go to the ice mountain and attack the ice demons there , lvl - 23 - 34.
read it too =D