Oke since I myself barly play Runescape I'm just making it member and gaining Loyalty Points and Coins. What I'm doing? Well it's not a big profit it'll take long...
The current price of the Spirit Shard is : 24 Gp
And you always can sell it for 25 gp at the summoning woman in Yanille.
So if you're barly playing like me, invest you're money it and then check over an week again you've bought quite much, and then simply sell it.
Not exactly a guide, but it helps quite a bit when you're not even doing anything.
You also only can buy 10,000 Spirit Shards every 4 hours, so it'll take long.