The board was borked when I answered this offline but the 2 main link rules are
1. No links can cross
2. You can't link FROM a portal under a field (but you can link TO a portal under a field, from one of the corners
Additionally:
3. Both target and source portals must be your team's colour, and have all 8 resonators
4. You must have a key to the target portal to link to it, and linking consumes a key
5. Links have range based on the portal level, 160m at level 1, over 1k for level 2 and so on but range can be extended with linkamps
It's a lot easier to experiment with linking in cities where the portals are close together. You can use the intel map (
http://ingress.com/intel ) to plan links and see what's going on around you but mainly on desktop as the mobile site is slow and buggy to the point of unusable at times.
In addition to double AP till the end of the month, they've now made it so if you glyph hack a portal (long press hack) and select "request key" (draw the little up arrow ^ glyph) while the screen says "command channel open", you now get 0-3 keys per hack, which is great for doing fields.