You'll need to equip armour and weapons that have a negative Magic attack bonus, such as platebodies and full helms - you should be aiming for a -64 Magic attack bonus overall.
Splashing is when a spell is cast but deals no damage - the idea behind this training method is that you can keep kitting the same NPC eternally if you never deal enough damage to kill it, meaning that this is a fully AFK way of training. Instead of killing enemies, if you want to brute force your way through your Magic training you can splash spells instead. Since these spells require Chaos Runes, they are more expensive, but this is still a relatively cheap way of training Magic. On reaching level 17 Magic you should switch to Wind Bolt and then proceed through the Bolt spells as you unlock them. You can mitigate the additional rune requirements of these elemental Strike spells by equipping a corresponding elemental staff.
When you unlock Water Strike, Earth Strike, or Fire Strike, you should switch to those for faster Magic training.
