Here is a quote from Nemgun:
https://bitcointalk.org/index.php?topic=229883.0quote:
"A block explorer is really easy to do, the main problem is about CSS, but for the rest, you can walk on it without any trouble.
First : You need a full node.
Second: You have to create some classes to retreive the block hash, block number, Block timestamp, TXs in block, Total fees, total amout of TXs.
Third : You have to provide a link where the users could click on Block hash, or TX id, and retreive its informations in the page.
These are the basics of a block explorer i guess.
In order to do that, you need :
1- Fully synced Daemon (not wallet)
2- Knowledge about RPC API of the daemon.
3- Knowledge about the language you want to use, either PHP, NODEjs, or any other language (any language can be used because it is a common API that will be used).
4- Time and patience, especially if you are learning.
I suggest you to do it on your own, it is better then use a ready made project, unless you want to save time, but you own't learn much doing that."
endquote