I was in the supermarket yesterday after lunch-time and had not actually eaten yet so... seems I went on a morcilla buying binge. They carried 3 types: Onion, Rice & Morcilla de Burgos. The Burgos seems to be an onion-based one but perhaps someone can enlighten me to the difference.

But, having never actually cooked morcilla myself before I was just guessing about how to do it.
I simmered them in water for a while; sliced into rounds; then sauted those lightly to crisp them up.
Seemed to work pretty well - - but really

what's the correct technique to prepare morcilla - solo.

Morcilla de Burgos with onion and rice, you cut it up and fry it in little olive oil. Do not boil it first, you lose the flavor. Spanish sausage is different from English sausage. The other kind of morcilla, you take it out of it's sausage wrapping/casing and you fry it up in a little olive oil with pasas (raisins) and pine nuts. I like to cook them until they are a little brown and crispy on the outside. I hope this helps!

I agree with Ingarrison. Definitely no need to boil the morcilla first. With the other dish (with the pine nuts and raisins) you can scramble some eggs and add them to make a "Revuelto".

Another kind of morcilla is Asturian morcilla which isn't very good for frying but ideal for making an Asturian bean stew "fabada".