Almost anything can be used as a mold, but plastic and glass items work best. You can pick something big enough to hold a few cups for one big bath bomb or use smaller molds for little bath bombs.  Plastics can soak up undiluted essential oil, but this is less likely to happen after you've mixed everything together. The most popular "mold" is a spherical plastic Christmas ornament. Look for the 2-part, snap-together type of ornament commonly sold at craft stores. It will make the round, softball-sized shape often seen in commercial bath bombs. Chocolate molds come in many cute shapes that are perfect for making bath bombs. Tart and cupcake tins also work well. You don't have to use dyes right out of the box. Try mixing them together to create you favorite colors.  A bath bomb that looks pretty when you make it may not necessarily make the best-looking bath. Keep a record of which combinations you've tried and which work out best. Make sure to always use dyes that are non-toxic, non-staining, and water soluble. Get creative with your bath bomb's scent. Blend different oils together to create your own unique aroma. If you don't know where to start, look up essential oil "recipes" online for ideas. Some popular combinations include:  4 parts spearmint to 1 part patchouli 2 parts orange to 1 part vanilla 1 part patchouli to 1 part cedarwood to 2 parts bergamot 1 part peppermint to 1 part tea tree to 2 parts lavender Equal parts lavender and peppermint

TL;DR in Chinese:
挑选模具。 选择和试用各种颜色。 寻找合适的香味。