maybe later will be renamed Law and Order universe, but I am watching Chicago series right now...
update: sorry but Chicago Fire and PD only except the crossovers bc the Med and Justice series not my cups of tea, maybe later I will update this list or feel free to suggest where to insert those episodes.
my watching order is focused on grouping series instead "real" timeline by switching between series, so you can binge-watching one series till crossovers.
CF s01 - Voigh (s01e02) and Antonio (s01e06) of later show CPD are appear through the entire season
CF s01e23 - Chicago P.D. backdoor pilot
CPD s01e01 - preimere
CF special? - I am a firefiter webisode series
L&OSVU s15c15/CPD s01e06 - Lindsay and SVU solving a series of rapes and murders: two parts crossover
CF s02e20/CPD s01e12 - major explosion at Chicago Med: two part crossover
CPD s02e03 - Sylvie Brett pops up (the new character was introduced in CF s03e01)
CPD s02e04 - Mills already in new position (resolution of CF s03e03)
CF s03e07/L&OSVU s16e07/CPD s02e07 - CPD and SVU investiage a child pornography ring: thee parts crossover
CF s03e13/CPD s02e13 - more clues about Shay's death: two parts crossover
CPD s02e18 - Will Halstead is yet to decide to work for Chicago Med, so before CF s03e19
CF s03e19 - Chicago Med backdoor pilot
CF s03e21/CPD s02e20/L&OSVU s16e20 - apartment fire and case of attempted rape and murder : three parts crossover
CM s01e01 - premiere
CF s04e10/CM s01e05/CPD s03e10 - Herrmann is treated at Chicago Med, chemoterapy fraud: three parts crossover
CF s04e11 - Hermann gets back to work
L&OSVU s17e14/CPD s03e14 - New York hunt for two escaped murderers: two parts crossover, connected to last season crossover events
CPD s03e21 - Chicago Justice backdoor pilot, L&O guest characters
CPD s04e01 - Julie's first day at CPD
CF s05e01 - Burgess and Julie works together
CPD s04e05 - Julie's last day at CPD
CPD s04e08 - Antonio's last day at CPD
CF s05e08 - plot starts with Severide that leads to the crossover
CF s05e09/CPD s04e09 - Severide's car caused a deadly crash: two parts crossover
CF s05e15/CM special?/CPD s04e16/CJ s01e01 - warehouse fire, Olinsky's daughter: three (four?) parts crossover, Justice premiere, Chicago P.D.'s Antonio already works with CJ
CJ s01e02 - Law&Order and Chicago P.D. guest characters
L&OSVU s19e13 - Law&Order and Chicago Justice guest characters
CJ s01e13 - last episode
CPD s05e16/CF s06e13 - serial bomber: two parts crossover
FBI s01e01 - premiere
CF s07e02/CM s04e02/CPD s06e02 - apartment complex fire: three parts crossover
CF s07e15/CPD s06e15 - firehouse lockbox key robberies: two parts crossover
FBI s01e18 - FBI Most Wanted backdoor pilot
CF s08e04/CM s05e04/CPD s07e04 - Infection: three parts crossover
FBIMW s01e01 - premiere
CF s08e015/CPD s07e015 - solve the case of a bad overdose: two parts crossover
FBI s02e18/FBIMW s01e09 - two parts crossover
FBI s02e19 - Chicago P.D. characters pop up
FBI s04e01/FBIMW s03e01/FBII s01e01 - three parts fbi crossover
FBII s02e18/FBI s05e19/FBIMW s04e18 - three parts fbi crossover