Items needed

Sub and enclosure
Amp-Sub phono cables
Amp wiring kit (1x12v,1xearth,1xremote,2xRCA)

Tools needed

Head unit removal tool
Screwdrivers,phillips and flat blade
Tools to disconnect battery(varies from car to car)
Precision termination tools (crimpers or pliers and thin in-line splices)
Soldering iron could be used as an alternative

1.Start by disconnecting the battery on the positive side(always do this when working on a cars electrical systems)
2.Run the 12v cable from close to the battery,through a grommet on the engine bulkhead(move from car to car,on escorts there is on behind the wheels behind the arch liner) through the car(down one side of the car under the carpet/trim) and then into the boot,connect to + terminal on amp.
3.Run the earth cable (brown or black and not very long) from the negative/earth terminal on the amp to an earth point in the boot (try around the rear light cluster as they have earth points there from factory)ensure it gets good metal on metal contact
4.Connect remote lead (thin blue wire)to the remote input(REM) terminal on the amp and run through the car,follow the red cable you just laid) and take it under the dash and into the back of the head unit (check your head unit instruction manual to find out which wire to splice it too,often a blue wire)Use crimping tool(or pliers)to crimp an in-line splice onto the 2 cables to make them 1,alternatively you can solder the 2 cables together and cover with insulation tape.
5.Connect the RCA cables to the back of the head unit (RED on RED,WHITE on WHITE) and run them down the other side of the car to the power cables until they enter the boot,connect them to the input side of the amp,RED on RED,WHITE on WHITE. Next connect the short RCA cables from the output of the amp to the input on the side of the sub/enclosure
6.Turn all the gains on the amp right down,connect the battery back up,ensuring you connect your new 12v amp cable,and ensure the in-line fuse holder has a fuse in,then turn the head unit on,the amp shud come on when the H/U comes on.Set the head unit to a comfortable level and go to the boot,tune the gain(bass boost) and frequency range to your liking
7.Secure the sub and amp to the boot floor and tidy the cables under the carpet and your done

Key to diagram
1=12v Power cable,from amp + to battery
2=Remote lead,from amp remote input (REM) to head unit remote wire
3=RCA through car,into Head unit RCA (keep clear of power cable)
4=Earth,from amp -(NEG) to earth point,rear lights?
5=RCA in,connect these RED-RED WHITE-WHITE
6=RCA out,connect RED-RED WHITE-WHITE from amp out to Sub

Diagram of boot,bottom of pic is entrance side of boot

Laymans terms for some things
RCA/Phono leads.Chunky cables that carry the audio signal from head unit to amp and amp to sub,have red and white connectors on
Head unit keys.Vary from head unit to head unit,a standard head unit in most cars uses a two prongs,looks like this

Problems you may find

1.Amp does not come on=Check remote feed,and power/earth
2.Volume/bass very low=Turn boost up,check head unit settings and increase Sub+ if feature available
3.Amp cuts out=Lower boost and ensure amp well ventilated
4.Sub pops=Lower boost
5.Whining noise audible=ensure RCA leads are clear of power cables

That should help everyone wire a sub up safely and servicably

Finished,all my own writing,info taken from my knowledge only