DrumSetSheetMusic.com is both an index (charts are hosted on other sites) and a marketplace (charts are hosted here on site). So the process is slightly different depending on what song chart your looking to download.
When you arrive on a song product page if the chart is natively hosted on this site then you will see an "add to cart" button.
After adding the chart to your cart, creating an account, and purchasing the sheet music, there will be a download link on the order confirmation page. Additionally an email will be sent to the email address provided during account creation.
If you use the link in your email, it will take you to the order confirmation page with the download link. Keep this email, as the download link will not expire, and you can download as many times as needed.
When you arrive on a song product page if the chart is hosted elsewhere the "Add to cart" button will instead display a "View chart on ... .com".
After following the link it will either take you to a list of songs on which the chart you'll be looking for will need to be searched on, or it will take you directly to a product page. Either way the download process will be different for each site, so you'll need to reference their FAQ page, How to Download page, or their Terms of Service page.