Dark retreats are only possible at times when a Lama is in residence in Yeshe Sal Ling - see the schedule, and only on a Lama's (preferrably your root Lama's) advice. It may be possible to invite a Lama to be present especially for your planned dark retreat, if you ask about this early enough. The special invitation of a Lama to supervise dark retreats is most likely to be successful if two people plan on doing dark retreats at the same time.
In general, longer dark retreats have to be planned at least several months ahead and you will need the advice of your root Lama who will tell you how to prepare. For instance, for the classical dark retreat of seven weeks according to Gyalwai chaktri, it may be advisable to learn some trulkhor cycles and other auxiliary practices by heart, all of which takes time.
After the end of a dark retreat, allow for a few extra days (or longer in case of a dark retreat of several weeks) for getting accommodated to light again and then cleaning the room (which is likely to be a mess) for the next inhabitant.
There are two dark retreat rooms in Yeshe Sal Ling, both spacious enough to do trulkhor, and equipped with all modern conveniences, such as a shower and central heating. As of April 2016, we can say that the first dark retreat of one week has been successfully completed under Lama Sangye Monlam's supervision and that the retreatant was happy with the experience.
Personal retreats for the practice of Bön can be undertaken also at times when no Lama is in residence, although it is of course a good thing to do your retreat when a Lama is present, because of the advice and inspiration by the Lama. Retreats for technically challenging practices such as tummo should only be undertaken at times when a Lama is at hand to give support and advice in case of difficulties.
The periods when Lamas are in residence at Yeshe Sal Ling are posted on Yeshe Sal Ling's schedule.
Personal retreats, too, should be planned ahead, and your intention announced early, to make sure that a room will be available and some arrangement for food (cooking and shopping) can be made.