It's very easy. Open the property box for the individual and add the first spouse in the spouse box. A tab appears with their give name on it and next to this you will see a little "+" tab appear. Click on this and you can add a new spouse and children. If you then need to re-order the marriages you can do so by clicking on the arrow to the right of the spouse box (which appears once multiple spouses have been entered).

You can also add a spouse or other individual by having the relevant spouse or parent etc open in the focus window or property box and click on "ADD" on the very top menu bar.

There is info in the HELP section on FH under the How To section. If you haven't been through the Onlune tour of FH or the getting started sections in help it's well worth doing.