There are some skills that are much easier to get to maximum level than others. Prayer in my opinion is by far the most tedious and lengthy. Bones only give you a small portion of prayer points when you bury them and when using prayer those points deplete quickly. The experience gained through this process is so minuscule that you feel like the experience bar hasn't even moved! Honestly, you may want to use the bones for something else, like the multiple potions that require them. I suppose you could fight things for days-on-end to get bones you need to start prayer, but there are much better methods that don't feel wasteful. Try this method and you won't be disappointed.

First thing to do is to forget about prayer. Literally don't waste your time on it. The reason why is found in how prayer works. You get experience depending on the damage you do. If you hit harder you get more experience. Instead of working on prayer, focus on completing the Volcanic Cave (melvor wiki can help you with what is required to AFK this dungeon.) Once completing this particular dungeon 100 times, you have now opened the ancient magic spell-book.

Next you need to build up your prayer point stash. Start casting 'Blessed Offering' under alt magic. Doing so will give you the holy dust which can be buried for prayer points. You will want about 200-300K. Another alternative if you didn't want to use alt magic is Fishing raw skeleton fish. it's not as fast, but just another alternative to slaying monsters.

Next step is to gather Fire Runes and Ancient Runes. You need both of these to cast the newly acquired ancient spell 'Ignite'. This spell costs 20 fire and 5 ancient runes. The amount of fire runes can be significantly lowered by spending some time getting the highest upgraded fire staff or fire wand; I highly recommend this. The reason 'ignite' is so important is because every time you hit something, (which it has a high accuracy rating) you will deal 680 damage; no more; no less. Prayer experience is determined by the amount of damage you do per action cast. Other ancient spells are not advised because they will cast a spell multiple times per action and each cast within that action will use up your prayer points... So don't waste your time. Only use 'Ignite'.

The monsters you want to kill are the high HP ones. Activate the 2 highest prayer skills you have unlocked, (regardless of ability and combat style). For example, If the highest prayer skill is one of the 'eye' spells that boosts ranged stats, use it; the perks don't matter. The cost of the prayer spell is the key.

**CAUTION: Be sure to read the prayer skill! Some of them do not grant any experience.**

Other tips that may help:
If you are focusing on high HP monsters, you could start thinking ahead and kill the things that drop higher tier gear such as the Pegasus for claw fragments, or monsters that drop the 'zodiac' necklace pieces. Most importantly, Just enjoy this tremendous game. Make it yours and play how you want. This is simply just a guide from someone who has been playing since its release. =D