From roo'ing the fleece will probably not be whole. Sort through it all - some of this can be done as you roo, so you discard any bits not suitable for spinning, felting or whatever. This includes any bits contaminated with faeces or heavily contaminated with veggie bits ie hay, burrs, bits of thistle and so on, plus any very coarse bits. If you want to use the coarse bits for another project then put them in a separate pile.
A Soay fleece is not usually very large, so you can often wash the whole thing in one go in a 3 gallon bucket.
Fill the bucket with hand warm water and a good squirt of your cleaning agent - this might be 'woolwash', Ecover washing up liquid, or a special wool scourer from a spinning supply shop.
Dump the fleece on top of the water and let it sink in itself. Never agitate the water or rub the wool, just let it rest for the first wash to loosen the dirt. Tip the bucket upside down somewhere like on a path to remove the wet fleece once the water looks wonderfully filthy. If you have a free standing spin dryer these are great for getting the water out once you've tipped out the fleece.
For your second wash you have a choice of another hand warm wash, which will remove dirt but leave some of the lanolin in the fleece, or you can use a very hot wash to remove every spot of grease (you need to do this if you intend to dye the wool) With a hot wash you must be even more careful not to agitate or rub the fleece. For this wash I gently and slowly squeeze the fleece, as it lies in the water, to get the last dregs of muck out. Tip it out or spin as before, then prepare a bucket of rinse water. This should be at a similar temp to the last wash water, although in practice I find the fleece will have cooled down after tipping out. Repeat with a second rinse if the water is still soapy.
Lay the wet fleece out on an old towel, either outside if it's not windy, or on a flat dryer indoors somewhere warm. leave it to dry then get going with your crafty project.
The aim of the care you take is to prevent the fibres from felting together. The conditions which aid felting are water, heat, soap, agitation and whacking (when making felt one stage involves slapping the partially felted fleece repeatedly on the table) so tip your bucket up gently.
HTH