replacing portion of roof

I will be re-flashing a chimney and consequently will need to replace a portion of the surrounding shingles. What is the procedure for properly weaving new shingles into old? I will need to remove the old asphalt felt and weatherguard near the chimney = is there anything special to keep in mind when laying the new felt / weatherguard next to the old? The more uncertainty I can remove at this stage, the smoother the job will go. Just for reference, I will be replacing about 3 squares worth of roofing, it will go from the gutter to the ridge, but will abutt existing shingles on one side.

thanks for any input you may have

 

Where the new shingles meet the old, carefully remove full shingles. Then weave in the old, beginning at the bottom edge and working toward the ridge with each course of new shingles.