I did something similar a while back.
Call your local rental yard and see if they have some adjustable jacks which will reach from ground to underside of porch. (There quite heavy so you may need an assistant), If you can't rent them then the only company I found that carry various jacks (have to buy) is Ellis OK
Depending on your ground (hard / soft), cut a couple holes through the porch floor (I didn't have to do this) and place either 3/4 inch plywood or steel plate (1/2 or thicker) on the ground. The larger the pieces the better to distribute the weight. Then get some 4x4 or 4x6 (depending on whats on the roof (mine was tile and I used 4x4's with jacks spaced ever 4 feet). Attach the 4x4 to the top of the jacks
(they have screw holes) and lift jacks (two at a time) and beam into position. Alternatively, (I found it easier) to use two 10 foot step ladders, placed the beam on top of the ladder, stand the jacks up under the beam. attach the beam to the jacks, and then screw up the jacks until they touched the underside of the trusses. After all jacks are in place (touching the bottom of the trusses), screw up each jack
1/4 to 1/2 turn until roof is supported on the jacks. then replace the support columns so they again rest on the original pier supports.