Step 1) Check that your inverter and the battery are the same voltage.
Step 2) Inverters, (every one I bought) comes with the correct cables. Check the size of the holes an the lugs and compare it with the studs on the battery.
Step 3) Check the at the invertor main switch is in the off position.
Step 4) Fit the black one and tighten the nut tight, but not near breaking point.
Step 5) Do the same with the red one. Be prepared for A BIG SPARK. IT IS NORMAL.
step 6) Switch the inverter on, but without load. All OK? Then ad the load.