In general the process is:
1.) Collect encrypted engrams. These can be from regular strikes, patrols, PvP, you name it.
2.) Put on your absolute highest light score equipment, even if it isn't what you normally use.
3.) Decrypt one item. Check if that item raises your light score. If it does, equip it. Proceed to decrypt the next item and repeat this process until all items are decrypted.
4.) Go re-equip whatever it is you would actually use out in the world.
5.) Go back to step 1.
The game decrypts new items based on your current light score, so just repeating this process will raise your light score very quickly. I went from 219 (right when I hit Level 40) to 290 in about 8-10 hours, and I wasn't being efficient for a lot of that given I was avoiding quests so as to stay with my friend progress wise.
Side Note: Go talk to Xur in the side room in The Reef and see if there's any exotic item you want. Remember, you can only have one exotic weapon and one exotic armor piece at a time.