Eggsish:If you play through the game on easy and take your time and look for them and kill the things required for the skull to appear it's pretty easy. Some black boxes may be a pain to find, but just go around the map with a Hornet and look for it. Hornets are good for this because they're both fast can get anywhere.
Hornets are definitely the way forward to finding the skulls and black boxes as some of them can only be reached by air units, but also anything that can travel in a straight line over any terrain will be able to reach them quickly. Also please note that to actually see some skulls/balck boxes you WILL need to rotate your field of view from the riginal position as they can be behind or under scenery.
To pick them up send a unit to their location (you should get a square cursor around the object like when collecting crates.
Black boxes are literally a black and yellow box about the size of an infantry man (but a little wider) and are usually either visible from the outset or if hidden under the fog of war quite often near some visible identifier (e.g. steam, flaming wreckage etc)
Skulls are identified when you complete the specific secondary mission on each level (usually kill x number of specific units). THEY DO NOT APPEAR UNTIL THIS IS DONE! When you're about to complete the objective look at your mini map as a flare (green pulsing circle) will identify the skull's location when you reach you're objective quota which makes them much easier to find than black boxes. A Skull is about the same size as a black box.
Hope that this helps (or if you want to cheat I've noticed a number of posts that contain the locations of all of these items...personally I found it more fun to go looking for myself as it meant that the whole map got to be explored as I looked for the balck box!)
I give no Mercy.
I feel no Regret.
That's the Truth.
I am the Hierarch or Hierarchs
I am the Prophet of Pain....