@Gundog covered the bases. @smalljaw67 also fishes the Susquehanna a lot. I grew up on the North Branch, and I still fish it a lot. @J Francho was also spot on about following Jeff Little. I haven't used him as a guide, but his instructionals on river smallmouth fishing are probably the best out there.
April into May is generally your pre-spawn into the spawn, and can be awesome for both numbers and size. In the spring, water temperature, river clarity, and flow determine pretty much everything and will determine whether you're throwing big spinnerbaits, a drop shot, or a grub on an 1/8oz head. I don't know the gauge around Duncannon well enough to be familiar with the levels, so reaching out to a tackle shop, guide, or someone who fishes from a local launch to get familiar with the levels would be a good start. Also, because you'll see a wide variety of water with fish holding likely in different places, prepare to do some junk fishing or jumping between patterns.
Smallmouth, in general, can move a lot, and that time of year water temperature and flow rate are especially huge. If the river is up and cloudy, you'll be finding fish off of points, on the tails of islands, and holding close to shoreline structure. Fish can be caught flipping jigs along submerged vegetation (not unlike fishing in the south for largemouth), or working the current seams along large current breaks with whatever the water clarity dictates. Fishing for smallmouth in chocolate milk-is water is very, very difficult even slow rolling a double Colorado, Rattle Trap, or a jig with 300 rattles on it. If you find dark water, look for creek mouths likely bringing in clearer water and fish the seams around them. If you're going to find active fish, that's your best bet.
If the water is lower/warmer, smallies will put on their feedbags and sit close to fast water along wing dams, boulder fields, tailouts, ledges, etc. These will be active fish and can be caught with 3/4 of your tackle box. Along faster, shallower water in the spring, it's hard to beat a good shallow/squarebill crank, tubes (I generally rig them on tube jig hooks ranging from 1/8-3/8 oz depending upon current and water level), 4" grubs (1/8-1/4oz), soft plastic swimbaits and soft plastic jerkbaits, finesse jig/craw, and even football jig/craw along gravel/weed edges.
As for presentation, I can make a bunch of suggestions. How familiar are you with river/stream fishing versus lakes? Are you familiar with presenting in current?
If you catch some rough weather, or have a tough time with reading a river/boat positioning/presenting in current it can be a frustrating place. If you just cover your basics, on the Susquehanna leading into the spawn, you could be in for world class smallmouth fishing.