Prayer is a notoriously expensive skill to train, and while the traditional method involves going into oter player owned houses, and using your dragon bones on their gilded altar, this guide will go in depth on how to get 99 prayer fastest, cheapest, passive prayer exp, and some alternative methods.
Traditionally, you want to train prayer by going into someones player owned house on the world designated for it. But there is also the wilderness altar, which has 50% chance to save a bone when used, and there are a few easy ways to avoid getting attacked (that much) when using the wilderness method. First off, using a high risk world. You dont really have any reason to bring a +1 as youre just training prayer, and the pkers have to bring their +1's since theyre skulled, so there is a bigger chance that the pker will avoid going into a high risk world. Personally, the best experience i have had using this altar, was on the high risk pvp worlds with a few sara brews, dinhs bulwark, and restore potions. Overall tanking gear. If you cant affoard this gear, either buy osrs gold and get the gear, or dont bother. An alternative to it is to train prayer as the first thing you do on your account, as a level 3 since a lot of people will not be able to attack you.
The fastest way that you can get 99 prayer would be using the expensive Ourg bones on a gilded altar, and at 75 prayer, switch to superior dragon bones, though this is not efficient when it comes to gp/exp, so you could essentially buy osrs gold, or use dagannoth bones to save gold when going for your desired level.
The cheapest method would be using big bones on the using regular bones on an altar. Even though thats slow, itd the cheapest gp/xp youre gonna get. Instead, you could use big bones which are currently around 0.60 more gp per exp, but you get a bigger exp drop. Another way to get exp would be using the altar in the wilderness to train you prayer. Even though this altar is in a multi combat zone, theres a chance that you could save your bones when using this altar, but since it is a PK hotspot, you are going to need some good gear to tank the pesky pkers. Currently, the meta for tanking pkers is 3-iteming your prayer in the wilderness, by using a dinhs bulwark, serpentine helmet, and a ring of suffering, along with a dhide set. These things can be substituted for better things, which you could buy osrs gold to reach.
The afkable method to train prayer is to get the bonecrusher from the morytania hard diary, and take it with you whereever you kill something that drops bones. A popular method when doing this is taking the bonecrusher with you while chinning in the monkey madness 2 tunnels, to get some passive prayer exp. Chinning is expensive, so you could buy osrs gold to get the desired stack of chins/barrage runes.
Alternative methods can be used to train your prayer in the wilderness, for example burying lava dragon bones on the lava dragon isle for the same exp that youd get at a gilded altar, or for ironmen to use the bone yard, or pest control for that matter.