So...let's get started! First off, you need to buy roughly 4,550 Rune Essences. Remember, don't buy Pure Essences, you will only need to buy regular Rune Essences. After buying these essences in the Grand Exchange. Type ::rc in your chatbox. You will be automatically teleported to the Runecrafting Skilling Area, as shown in the picture above. From there, walk to the East and find the Air Altar. Then, just start clicking on the circular stone of the Air Altar. You will want to make 2,293 Air Runes so you will get to level 65 Runecrafting to do Death Runes. Once you have level 65 Runecrafting go to the South a little bit and you will find the Death Altar stone. Click on that as fast as you can, until you make 2,249 Death Runes. You will then have 99 Runecrafting!
The main reason why I didn't switch back and forth from altars leading up to the Death Altar, because it would be most of a hassle switching back and forth between altars. So, I would just stick to Air and Death.
Runecrafting is surprisingly one of fastest 99's you can possibly get on Sage-Scape. Time is very hard to specify because it really depends how fast you can click on the Altar. I just got 99 Runecrafting in less than 20 minutes. So, if you want a fast 99, try Runecrafting!
If you were wondering about how much Experience you receive for each, here you are:
1 Air Rune: 196 Exp
1 Death Rune: 5,600 Exp
Thank you for looking at my guide! I really appreciate it and hopefully this helps!