General procedure
Large-scale facilities are not like an in-house lab where you can conduct a measurement "whenever you want". To gain access to experimental time, i.e. "beamtime", a proposal have to be submitted to the facility in question.
The procedure is very similar to a funding proposal. There are usually 1-2 calls per year with fixed deadlines. When the proposal is submitted it will go to an international review panel that will rank all proposals according to several criteria (feasibility, scientific excellence, potential scientific and societal impact, need for large-scale beamtime, etc.).
After 3-6 months you will receive an answer if your proposal was granted and how many days of beamtime you have been allocated. The beamtime will then be scheduled and you can travel to the facility to conduct your experiment.
This procedure (from submission of proposal to performing the experiment) usually takes 6-12 months, i.e. one has to plan ahead for projects involving measurements at large-scale facilities. In very high-impact and time-sensitive cases it could be possible to gain exclusive rapid access (e.g. so-called "Director's Beamtime" or fast-track proposals). However, this is very hard to get approved!
Procedure
Have an idea for how large-scale facilities can help answer your research question(s)
Talk to an expert (e.g. send me an e-mail: condmat@kth.se )
Consider your sample! (available size/mass, crystal/powder/thin film)
Think about if you sample contains elements with low scattering or high absorption: www.ncnr.nist.gov/resources/n-lengths/
Contact instrument responsible to discuss experiment (before you submit proposal!)
Write a proposal and apply for beamtime at your selected neutron source/instrument
Cross your fingers and wait for the review committee + in some cases ”national quota”
If you obtain beamtime start to prepare your experiments well advance (align crystals, sample holders etc.)
Check necessary paperwork in country + at source and perform the mandatory ”safety training”.
If you plan to do experiments at different sources with same samples: consider activation of your samples (active sample transport is complicated and expensive!).