You'll get these types of travel tumblers free at conventions, events, and gift bags. They're cheap, leak, spill, and don't keep your drink hot any better than a ceramic mug. Honestly I think my heavy ceramic mug does a far better job of keeping my morning coffee hot than these ever did.
While I do have great ways of keeping my beverage of choice hot or cold I did need a nice way to clean my brushes when painting. Now I have it and am actually looking forward to getting a few more of these throw-a-way give-a-ways in the future.
Step 1: Break It
That's really all there is to this. Take of the cap, this one actually has a decent gasket but that alone doesn't make it work, and use a tool to pry up the bottom half of the sliding "seal". Ugh. The only thing this type of design seals is old liquid that gets gross and is impossible to clean.
Step 2: Reassemble and Use
Add your cleaning fluid (water, soap and water, toluene, etc). For long soaking and solvents that evaporate quickly I cover the top with cling film.
Thinking about making a 3D printable cap to create a better seal.