the main problem with fitting is the cill.
you just have to be carefull when cutting the cill so cut a bit off at a time and keep checking.
you will use mdf for the internal cill counter levered so be carefull cutting that.
Use gallow brackets for under the cill this will stabilise the weight.I dont know if your using grp roof or deciding to cut lead but in any case if you dont have experience of doing this type of job I think you will have trouble
fitting it,I suggest employing somebody to do it for you.