a plug cutter is a little hollw drill bit type thing, you stick it in a DECENT cordless and fire it into a bit of scrap offcut, it leaves a little plug which you just snap out with a screwdriver bit like a mini dowel..
drill through your skirt (youll need to pilot drill oak anyway bout 4mm) then counterbore the hole bout 8mm deep..
send in your screw (use a No.10 i think cutter) and an 3"x8 screw..
it'll sit 8mm below the surface, then stick some glue on your plug and tap it into the hole, it'll go in tight fit but you need to use the right size counterbore drill, buy it when you buy your cutter theyre only a few quid each..
but and scribe joint...
send one side stright butt up to the wall, dont mitre it..
work clockwise round the room if your right handed or revese it if your left, youll see why when you come to scribe the skirting over the end of your hop up..
measure from the BACK edge of your just FIXED butted peice to the opposite wall, make it a good measurement..
get your next bit and cut a mitre on the left hand edge as though you were mitring the two bits.. cut the other end long (thats your next butt end)
get a coping saw and cut out the shape that the mitring leaves and offer it up to the butted peice.. make sure it fits (if its up square itll be bang on, always try and get em up square, this is what joiners moan about when the plaswter scoops out at the bottom)
if its right cut your peice square to length at the other end, if not adjust your chopsaw angle, zip it again, and do the coping saw thing again till it fits..
youll only get a double edge mitre to work if your rooms dead square and theyre quite long lengths..
if youve got external mitres to do, get them on first and work up to em..
glue and pin extenal mitres together before fixin em..