Measure the length of the roof and then add on about 15 inches,cut your felt to this size,roll out on lawn to cut, and always try to cut with either a hooked stanley knife blade,and always away from you.
Roll up this piece and place on roof,unroll from the end of the shed and slightly over lap,at the end,pull gently down to the gutter line, and then using clout nails, ensure they are short enough not to penetrate the roof,then nail every 4 inches along the gutter line,puulling felt gently, until you reach the other end,
Next pull the felt up and do the sme ensure that its tight,along the top, you can us eless clout nails here as it will be covered by the top caping layer,latter.
Now proceed to do the same the other side,then fit the top capping piece, and do the same the capping piece should lie equaly on either slope,now clout nail this all along the joins.
a few 2 x 1 treated batons down the slope will stop the felt lifting and ripping.
Best to get a freind to help you and dont try this on a windy day as youll rip the felt.
Sent you an email with some more info.