To monitor a squid access log, you should only need to set up an input, either via the GUI if the file is located on an indexer, or via inputs.conf if the file is located on a forwarder.
As part of setting up the input, you should assign a sourcetype. Hopefully, you can use an existing sourcetype. If not, you can define your own. Here is some information about sourcetypes (within the page you will also find a link to the "pretrained sourcetypes"). Why sourcetypes matter
Finally, once you have the data in Splunk, you can create a search and a graph.
If you are confused by all of this, you might want to walk through the Splunk Tutorial or one of the free video tutorials here.