I learned a lot from Susan at swiftcraftymonkey, but I also appreciate the concise, step by step information in Anne Watson's book Smart Lotionmaking. I think you can get it for a small song on Amazon as a download, but I bought the print book too. Anne describes two levels of sanitation -- the personal level (easiest) and the pro level (more rigorous). I'd start with the personal level to get started and then upgrade to the techniques she describes for the pro level if you eventually intend to sell.
I have not had any failures yet as far as the lotion separating, but I have made lotions I like much better than others as far as skin feel, ease of use, color, scent, etc. I think a month or two of regular use is plenty of time to evaluate whether I like a recipe or not. I would think that amount of time would show whether the emulsion would be stable as well, although I know cosmetic companies evaluate stability by putting samples in a heated cabinet for months to challenge the product under more extreme conditions and to artificially accelerate the aging process.
As far as product safety goes -- On an informal basis, yes, you can look for obvious fungal growth, but you can have high microbial counts without visible signs of contamination. The only way to really know whether your sanitation practices, packaging, and preservative system are effective is to have the product challenge tested. There are some do-it-at-home tests available, but the gold standard is to send a sample to a lab and have them do the procedure.
Sanitary vs. sterile -- Unless you are working in a sterile environment with sterile techniques, you CANNOT make a
sterile lotion. Even an operating room isn't sterile, although the instruments used may be sterile to start with, so there's no hope of working sterile in our kitchens! The best we can hope for is to make a
sanitary lotion -- one with microbe counts as low as possible at the time the product is made and packaged.
Trix mentioned the heat-and-hold step -- I agree it is really important. I also perceive many beginners want to avoid this hassle. There are some room-temperature emulsifiers that can be used to make lotions, but ordinarily the heat-and-hold step is not something I recommend skipping. It's what helps the lotion emulsify effectively (with most emulsifiers) as well as increases the sanitation level of your products. I'm all for making a lotion that is sanitary as possible -- then the preservative in my lotion will last longer and do a better job.
I actually think lotion making is easier than soapmaking.
There is the challenge in both to do things precisely and carefully, but there's more latitude in lotion making to try different oils and other ingredients.
The feedback from a lotion is more immediate -- no waiting for a month or so to evaluate skin feel and lather quality. That said, I have learned to let the lotion sit for about 24 hours to cool and stabilize before evaluating how it feels on the skin, how it smells, how it looks. At least for the ingredients I've been using, my lotions feel overly greasy when warm and freshly made -- although that doesn't stop me from trying them out right away!
Lotion making is fun, and I'm glad you're going to give it a go, Froggy.
ETA: I think it can be a strong temptation in lotion making as well as in soap making to go crazy with additives, such as honey, milk, etc. Keep in mind that additives add "bug food" and the more bug food you add to a lotion, the more likely you will need to use a higher % of preservative for efficacy and the more likely it is that the preservative may become ineffective over time in its job to control microbial growth.
Making Skin Care is a good resource for information about additives,
preservative efficacy, and making
lotions/creams.
