RESEARCH
Lonely Planet Trans-Siberian Railway Book https://www.lonelyplanet.com/trans-siberian-railway
Seat 61 Website https://www.seat61.com/index-mobile.htm
Choose route first. (Trans-Siberian to Beijing, Trans-Siberian to Vladivostok, Trans-Manchurian). Consider adding to the beginning of the trip or end of the trip by starting in Europe or take the ferry to Japan (from Vladivostok). East or West?
Decide on time of year (July for Naadam, January for Harbin International Ice and Snow Sculpture Festival, etc)
Plan at least 6 months ahead because travel visas take a couple months and when applying you must know exact dates and hotels.