What's your access to the pool like? Thermoplastic walk-in steps with a gasket? The dimensions listed (26x43) make me think this is more of a freeform pool rather than a straight wall / geometric shape, correct? IMO straight wall pools will be the easiest to square up and drop a liner into, although having multiple 'hands on deck' to assist in the initial dropping and squaring is pretty important.
One of the biggest thoughts here for me would be having accurate dimensions. Personally, if I had a dimension sheet that I was confident was still accurate and could order the liner off of, and had a couple (3 or 4) buddies that were available for an hour or two on the day of the install to square up, I'd tackle it myself. If I wasn't sure of the dimensions and had to re-measure, I would probably call in an installer. As Realdeal mentioned above, it just takes a minor mistake in measuring to eat the cost of an entire couple-thousand-dollar liner, and I'd want to put the responsibility of measuring on someone who's done it numerous times / has insurance against these kind of mistakes.