Various tips that helped me when I was starting out, in no particular order:
only use a scent oil containing vanilla if you also use a vanilla stabiliser, otherwise the soap will turn brown.
don't use anything cinnamon as that is an irritant.
don't use any fresh or dried ingredients like flowers or herbs before checking here as most will mold/go bad in your soap!
only use liquid colors for 1 color soaps. If you want multiple colors always use mica, as liquid colors will bleed.
if using mica, premix this with a little alcohol or your scent oil to prevent clumping.
for strong colors use a clear base. A white base + color will give you pastels.
spray any mold with fine detail with alcohol before you pour, to prevent bubbles messing up those details.
rule of thumb, if you don't know the exact percentages, is to keep any added liquids or oils (including scent oils and liquid color, no need to include alcohol) to max 1 teaspoon per pound/500g of base.
only use skin safe and soap safe scent oils and coloring.
if using the microwave, use a REALLY microwave proof and heat proof container and heat in bursts of max 30sec, don't overheat the base.
spray alcohol (90% or higher is recommended) between layers or on any embeds before pouring.
silicon molds are much easier to use than plastic molds.
you can mix soap bases if needed, for instance for a less transparant soap use a small amount of white soap base and a large amount of clear base.
yes you can remeld your soap if needed.
cut a soap loaf on its side to prevent some of the pressure on different layers, to prevent them breaking up.
if you buy a soap loaf mold, buy a fortified one: either thick walled silicon, or thin silicon plus a wooden box.
if you have a thin layer of soap where another color leaked a bit, you can wet the soap and rub it over a piece of terrycloth or similar to 'shave' off that layer.
silicon type molds are much easier to use than plastic molds, the removal of soap from a plastic mold can be quite hard.
if you want to measure the amount of soap that you need for a mold, fill the mold with water and measure or weigh the water. This will give you the approx. amount of soap needed.