I'm trying to install ceramic tile in a friend's bathroom, appox. 45 sq ft. The current floor in the bathroom is vinyl squares over the base cement. There is no subfloor or anything like that. The bathroom is in the basement. I tried pulling off the vinyl but it's leaving a thick yellowish glue behind. What's the best / easiest process to install ceramic floor over the existing floor?
Is the glue residue hard and rough or soft and gummy? If it's hard, a 6" floor scraper should break it loose, maybe with a heat gun or some boiling water to loosen it a bit. If it's soft and gummy, the scraper should get it by itself, or you can try some acetone or mineral spirits to dissolve it (make sure you ventilate the area well, and take frequent breaks).
Either way, tiling directly over a vinyl floor is not going to be ideal. The thinset will not key well to the smooth vinyl (and trying to rough it up will basically just get you to the point where you might as well scrape it all off anyway). Vinyl floor should NOT be considered a "floating underlayment", either, as a few posts I've seen have indicated. It's too thin, and actually MORE likely to result in cracking as tiles break loose and try to shift.
You really need to get that stuff off. It will not be an easy job, and if he's not paying you for the time he should be.
Scraping off the vinyl would be best. Another possibility, if you are OK with raising the floor and fixtures (including raising the toilet flange), would be to mechanically fasten down a tile underlayment over top of the vinyl. You could use a membrane like Schluter Ditra or a cement board-type product, then install the ceramic tiles over that.
You might also want to call up a flooring shop and see if you can rent a floor scraper-- that can make quick work of removing the vinyl, which is definitely the better way to go.