Where to write the java code to use custom share this button?

Hello User,

Please note that to add the custom buttons to your WordPress site, first you need to add the ShareThis code/buttons manually to your site.
You can know more about it from here –
http://support.sharethis.com/customer/portal/questions/943449-display-buttons-on-some-pages-but-not-all-pages#sthash.7vnoNk44.dpbs

Then you can edit the ShareThis code to add the Custom Buttons of your choice.
Please note that while adding the custom button, you need to add the below property in your website’s CSS property –

.st_sharethis_custom
{
background: url(“http://path/to/image/file”) no-repeat scroll left top transparent;
padding:0px 16px 0 0;
}

And you can find the complete information here –
http://support.sharethis.com/customer/portal/articles/446558-custom-buttons

Please feel free to contact us if you have any queries.