The first rule of making melt and pour soap is to use low heat to melt the soap, especially soap bases with additives such as oatmeal and milk products. This is especially important if you are melting soap directly on the fire in a pot. Then it is best to use a thick-bottomed pot, turn the heat down to low and add a small amount of water, say 1 tablespoon, to the pot before adding soap pieces.
After taking the soap off the heat, allow it to cool a bit so that a skin forms on the top, gently stir this skin back into the mixture and allow to cool until the temperature is lukewarm, at all times gently stirring in between cooling to get rid of the skin that forms on the top of the soap. This is very important. The cooler the soap, the more intact the final design will be and the more pronounced the scent will be because the fragrance will not be destroyed. I usually cool all my soaps until they are a bit warmer than lukewarm to the touch to spare the fragrance but also so that the design will not be shifted by melting pieces, and embedded pieces will not melt because the soap is too hot.
Use colorants that are non-bleeding to color embedded pieces and the greater soap design. Remember that bleeding occurs both from the embedded pieces and inward to the embedded pieces. This is okay though, if one wants colors that bleed. I remember the first time I saw melt and pour soap slices; the unstructured and fuzzy color combinations that enhanced its handcrafted look fascinated me. In retrospect, the soap I admired so much was actually soap that had the colorants that bled and disrupted the design, but I liked that as it appealed to me. Some color combinations that bleed into each other that are especially nice are blues that bleed into reds and yellows and greens that bleed into yellows and oranges. These are nice especially done in a clear base. An attractive design could include a fuzzy color combination with a dot of non-bleeding black or brown off-centered in the design –something akin to a modern dot of geometry in a sea of impressionism.
To layer soap—spritz between layers with alcohol or distilled water. If the soap you are making is for yourself and is to be used in a matter of weeks then, plain tap water is fine.
If embeds used are colored with colorants that are water dispersible and bleed, dip them or paint them in a solution of clear soap base. This will seal the outer layer and help to retard some of the bleeding of the colors. I have even seen soap pieces covered in a thin layer of wax to retard the bleeding between embedded pieces and soap.
Craft embedded pieces separately from the actual soap or soap loaves being made, and then have all the pieces laid out assembly line fashion to instill order and clarity to the project.
Have fun making soap!
Even though you are having fun, approach soap making with discipline. Have supplies organized and laid out for each step of the design process. Have a plan B for your design if that first design does not work out. This is especially important if you have to have a gift or order that is waiting for the final finishing touches and soap.
Try drafting a design ahead of implementation in a graphic design program. This gives you to opportunity to see color combinations together. One way to do this even if you do not have a graphic design program is to use paint chips to test different color combinations side by side.
Get ideas from different sources. Some of my favorite places to get ideas for design as well as color combinations are - greeting card designs, artful web designs, pattern books, graphic anthologies, fashion design color theory books, scrap making paper designs and the garden.