I make shampoo bars using a sous vide method -- in other words, I melt the ingredients inside a heavy weight plastic bag that's been placed in a hot water bath (bain marie).
I prefer to use bags meant for vacuum sealing (Foodsaver type bags) because they are a lot less prone to leak. Others use freezer weight food bags (Ziploc). The ingredients go into the bag and the bag gets partly submerged in the hot water that's just deep enough to cover the ingredients. I roll the open top over several times and close it with a "binder clip". That seals the top of the bag in case the top slips under the water. This bag method makes it easy to mix the ingredients -- I wrap the bag in a small towel and smoosh it with my hands.
When the ingredients are melted and well smooshed together, I lay the unopened bag on the counter and use a bench scraper (or the edge of a spoon or butter knife) to scrape the paste into the middle of the bag. This is much like pushing frosting around in a pastry bag. After the paste is in a tidy lump, I let it cool to room temperature. (I've also cooled the paste in the refrigerator and that works well.)
If you want to add heat sensitive ingredients, wait until the paste has cooled to the right temperature, open the bag so the lump of paste is exposed and add the ingredients. Close the bag and smoosh until well mixed* and then let the paste cool further to room temp or refrigerator temp. Be sure to allow enough time for the center of the paste to cool so the paste isn't quite so awful sticky.
When cool, I cut the bag open, peel the plastic back, and portion the paste into individual silicone molds. I press the paste with my fingers into the mold. After the paste is in the molds, I'll wet my fingers and smooth off the exposed paste so it's a little more tidy. Tap a little bit, check the underside of the molds to make sure they're not distorted, and done. I don't make bars for sale, or I'd pay more attention to getting the paste deep into all the crevices for the smoothest look.
I've also hand-formed patties if I didn't feel like messing with the molds. Freeze the bars for at least several hours, and then gently work the bars out of their molds. They usually come out fairly cleanly.
Another trick I think is helpful is to put the fresh bars on waxed or parchment paper or plastic food wrap and leave them in a frost free refrigerator (not freezer) for some days. The low humidity in the fridge helps the bars to dry out and firm up nicely. If your house air is fairly dry, just leaving them in the open would also work. Sometimes I make these bars in summer when the humidity is really high, and the only way I can get them firm and dry is to use the fridge.
Even after they've firmed up, I also store my shampoo bars in the fridge, because they seem more prone to absorbing water from the air than soap. I don't have to worry about the bars softening in humid weather if the bars stay in the fridge.
One caveat about fridge storage -- if you like to use fragrance in your shampoo bars, you might want to keep the bars in an airtight container if refrigerating them for a long-ish time. I have a second fridge in my pantry where I keep essential oils, short-lived fats, garden produce, pop (Midwestern US lingo for soda), and adult beverages. I could only smell a tiny hint of scent from the unwrapped shampoo bars, but that was enough to taint the flavor of lightly wrapped foods (bread, cookies) stored in the freezer part of the fridge. A lesson learned the hard way.
* One way to ensure last-minute ingredients are well mixed into the paste is to add a small amount of colorant to the paste along with the additives. I often use ultramarine blue, but a mica would also work nicely. When the color is evenly distributed throughout the paste, it's a safe bet the other additives are also mixed in well.
