Typically, there are two options:
- Combining a spotting scope with a digital SLR camera body. This costs thousands of pounds, and sometimes means you can't use the spotting scope alone.
- Combining a spotting scope with a standard digital camera via a dedicated adaptor. Again, there is cost involved - most adapters cost several tens of relevant currency units.
Step 1: What you need
Myself, I am using an old Opticron HR80 straight-through spotting scope (second hand, after my dad updated to Swarovski optics, flash devil), and a "Trust" webcam, which I bought because I thought I ought, and never used.
The attachment is merely a pair of rubber bands, knotted in a particular way, with an optional modifier made of a piece of card.