1st order, NEVER EVER leave the Cell Harness attached to the BMS when connecting leads to the cells. Instant POOFDAH with the slightest of oppsies.
2nd order, Electronics do NOT like Static Electricity ! So many people forget that and fry their stuff... it is sad because it is so stupidly obvious.
3rd, ALWAYS verify the BMS leads from cell to Big Connector are in correct order/sequence and use a DVOM/DMM (Multi-Meter) to ensure the connection is good.
BMS Connections VARY depending on type & class of BMS... Same-Port, Separate-Port, Relays/Contactors or FET Based only. IT HELPS TO KNOW WHAT BMS YOU ARE TACKLING ! Even some FET Based BMS has a place to run the BATT POSITIVE through the BMS for the internal voltage sensing (as opposed to sensing from the BMS harness which is less accurate) but mix them up and POOF magic smoke appears and the crying begins.
The BMS Harness is just that. It connects the BMS to the cells to monitor independent cell voltages. "Some" BMS have Passive Balancing which is in the milliamp range up to 1.2A max (across the whole balancer) and that can occur over the standard harness. Very Few BMS' have Active Balancing which can move up to 10A (typically < 5a) across the balancer and that would have heavier gauge wire as well. Often people will install an Active Balancer as a separate device when warranted. Generally, with properly Matched & Batched cells this is not needed as Passive Balancing is usually adequate. Bolk/Commodity cells can be troublesome with runner cells and imbalance issues and often an Active Balancer cam "Compensate" for such to get better results from your cells.