Petticoats are very complex pieces of clothing with many layers and a lot of fabric. They come in all shapes, sizes and colours and each petticoat can have an entirely different purpose. Petticoats are formed in two directions; layers and tiers. 1 layer can consist of many tiers, the more tiers the more flouncy your petticoat will be, the more layers the bigger it will be. Each tier should be double the length of the tier before it- if your top tier is twice your waist measurement then the second tier should be 4 times it. You can of course make you tiers longer or shorter but this can lead to a lack of support making your garment ineffective.
The standard 60’s swing petticoat will probably consist of 1 tier in 3-4 levels. The diagram below is a side cut of such a petticoat. The top tier is a single layer tulle or sometimes cotton tier, the second/third/fourth tiers are double layer tulle making the second tier 2 layers thick and the third tier 4 layers thick. Ball gown petticoats can be made in a similar way, but they need multiple layers in the above form in order to hold the fabric of the ball gown. Therefore many find it easier to make multiple single layer petticoats (without the doubling of layers at each tier) or single tier petticoats reinforced at the hem with horse braid hair or a crinoline underneath. However you choose to make your petticoat you should always start at the bottom and work your way up, first hemming the bottom tier, then gathering and attaching it to the next tier. Your bottom tier can end up being extremely long but you just need to persevere. I recently made a rainbow petticoat whose hem totalled over 60m over the 5 layers. Having that much tulle spread over your work space can be intimidating but you just have to remember to try, and that you can always leave it and try again another day.