Smart Place
Most of the time, items can be directly equipped where you want them. But some times it is possible to equip an item only by juggling some items or even weapons around on the mech.
LSML makes this process easy with "Smart-Place". When an item is not directly equippable, but might be possible to equip by shuffling stuff around, the item will be displayed as partially gray, like this:
Double clicking on such an item will first ask if you wish to use the Smart-Place functionality at all, and then if you agree, it will try to find some way of cramming the item onto your loadout.
This is an easy way of building a loadout. Simply double click what things you want to equip, look at the stats too see if it's any good. Remove items and keep adding new ones by double clicking until you're happy with the performance "on paper". Now things might be a bit messy on the loadout after doing this but it's fairly easy to just move things where you want them now that you know what you want.