Clerics and Druids however, cast their spells essentially through prayer and meditation. In addition, they use "offerings or sacrifices the same way that Magic Users use spell components. These offerings range from leaves or a holy symbol, etc...
So, we know how Clerics and Druids cast spells. We also know that they are limited in the number of spells they can cast in a given period of time due to experience level limitations. Another thing we need to know is how exactly do they get the spells they need to know. The above answer of using prayer or meditation only answers it so far.
Do they just pray for something in general or perhaps specifically that they want to have happen and the powers that be interpret what they want into the identified spell and make it happen? How do they know which spell they can use what they can do, at any particular level? Are they given spells by other, senior, Clerics or Druids during training?
I have answered these questions for Druids in my gameworld by using Druidic Rites of Ascension. Due to their typically solitary nature, it is unlikely that Druids go to Druid temples or training camps to get to the next level and learn the spells they need to know appropriate for their level or capability.
I have it so that when a Druid PC has reached enough XP to level up, they must go through the Druidic Rites of Ascension. Essentially a complex ritual that lasts for days during which they are given by the powers that be the information they need to know to carry out their duties for their new level.
The Druidic Rites of Ascension procedure is as follows;
- The ritual takes place in a sacred place outdoors.
- The Druid fasts for the number of days of their new level during the ritual.
- The Druid makes an offering or sacrifice of the size, or number of their new level at an outdoor alter. (I allow for anything that was living but died of natural causes to be collected and worked into a special form or design such as carvings made from deadwood of fallen trees, figures made from cornhusks and other crop plants, etc... The point is they were living, they died naturally or as a matter of natural events and the Druid must use craftwork or some special preparation or presentation of the material prior to being considered a worthy offering or sacrifice.)
- After the offering or sacrifice, the Druid sits in one place and prays/meditates on his/her duties and obligations as a druid, waiting for "Divine Inspiration" to be achieved proving them with what they need to know. The knowledge will consist of:
- The specific spells in the Players Manual for the level of Druid to be attained
- The knowledge of how the spells work and what they do.
- The knowledge of which, how much, etc... offerings (mistletoe, holly or oak leaves, etc...) that correspond to each spell.
The DM can do a lot of story-building with this ritual and it's purpose. I like to run it as a solo mini-adventure for druid PC's because I also give a substantial number of XP just for going through the process.
I also like to punish/reward the Druid for how well they have taken their Druidic obligations through previous activity. Have they been behaving/roleplayed ideally? The more true to character they are roleplayed, the swifter they can be rewarded. For example, if the character has been portrayed in a way that ideally reflects a Druid and honoring their druidic obligations, the wisdom they seek can be achieved after the first day of the ritual, minimizing time spent performing the ritual. Depending on the number of days planned for the ritual Good roleplaying can be rewarded by ending it sooner. Poor roleplay can be "punished" by making them go until the very last day. Make them earn it, I say. They work for Nature, the originator of "Natural Selection" which is the brutal and uncaring process of successful evolution and adaptation for progess and survival. Nature won't let a Druid off easily unless they have demonstrated the necessary skills. The DM should keep that in mind as they roleplay Nature's Will during the ritual.
There is the possibility that the ritual will not be successful. Mostly depending on how well they have been roleplayed. This can be addressed by the DM making a determination ahead of time of "Grading" the way the PC has been portrayed as a Druid. (remember DM's, you don't have t like their PC's personality, methods and means of carrying out their druidic obligations. The single important factor is determining how well they have been portrayed as a Druid and meeting their druidic obligations.) I use a simple 1 to 4 method. 1=meets all obligations (90-100% of the time), 2-meets most obligations, (70 to 90% of the time) 3- meets some obligations (50 to 70% of the time) and 4= meets few obligations (anything less than 50% of the time).
Based on that score, the potential success of the ritual is 1= guaranteed to be successful,,, within first two days. 2= High probability of success 80% on second day or later (roll percentile dice for probability 20 to 100 on percentile dice). 3= average probability on third day or later (20 to 70 on percentile dice). 4=low probability on 4th day or later. (20 to 40 on percentile dice.
If they are not guaranteed to be successful on the first day, then the percentile dice are rolled starting the day the probability begins. If they are low level and their ritual is only say 3 days for 3rd level and they have a 4 score, they are guaranteed not to have a successful ritual this time. Have them focus on living up to their druidic obligations and try again some time later.
The result afterward is having competent Druids who know what they need to know, when they need to know it. At least in terms of abilities and spell-casting. They might have an idea of what to expect by adventuring with and observing other, higher level Druids but those Druids would never overstep their bounds and interfere with Nature's methods in advancing other Druids.
I mentioned that I give XP for the ritual. I base the XP out of a total of one fourth of the points needed to advance to the next level. Then from that number, I give them up to a fourth of those points for how well they prepare for the ritual and for how fast they complete the ritual. The shorter the time due to good roleplaying, the higher the XP.
In regards to preparation, since we have gone to the trouble of roleplaying the ritual out, then it behooves us to see the whole thing through. If when they make their Druid at the beginning they are informed properly by the DM of the whole druidic ritual of ascension process, they should know what they need to do to prepare for it and describe each step of the process as they engage in it to the DM. If they don't know or don't remember, the DM can coach them through it or have a "guide" appear to show them the way.
One other touch I add is that the force of Nature itself marks each successful ritual by leaving what appears to be a brand in the shape of a star or a crescent moon on the Druid's body. I let the dice tell me where Nature puts the first mark (face, chest, arm, leg) and then the rest that are accumulated automatically are placed with it going forward.
That is the long way for the purpose of a solo adventure for the ritual. The short way is that the DM just makes the determination of druid obligation roleplaying and scores it appropriately. based on the score, roll for probability of success, if necessary and inform the Player of the results. No XP should be given for going the fast way. The Player should still be the one to initiate the process of the ritual, roleplayed or fast method.