When mixing your own potting soil there are a few things to consider, not all plants like the same soil.
There are plants that like an acidic soil, there are mixes for the propagation of cuttings, cacti and succulent mixes.
These mixes all have there own unique qualities for the specific purpose they are used.
For the propagation of seeds the soil mix doesn't need to contain any nutrients.
Seeds contain enough of the nutrients needed to supply themselves with to support there germination.
It does need to be a weed free soil, free of toxins, be able to hold moisture well and have plenty of air space along with being well drained.
Garden soils do not have these same characteristics, they tend to compact which makes it to dense for tender seedling root systems to penetrate easily.
To germinate seed, you can use soilless products like straight vermiculite, peat moss, sphagnum moss or perlite, but once your seeds germinate and the seedlings emerge you can create a soilless mix that will benefit the plants better by adding compost as an ingredient in your mix.
Research the plants that you want to grow before you start to put together a potting soil mix.
Potting soil is not only used for your indoor plants, it is also used in your outdoor container plantings and window boxes for flowers, herbs or vegetables.
Putting together the perfect potting soil mix for your gardening needs will take a little trial and error.
Every gardener has there own personal preference.
Try a combination of a few different soilless products and see what works best for you.