Here are some fancy hand-drawn diagrams.
For the prominent seat alteration:
1. Extend the crotch seam at the inside leg.
2. Slash at one or more points from the center back seam to the side seam and spread to add length to the center back seam.
3. If there is excess fabric in the front, remove length from the front crotch seam at the inside leg to match the back alteration. Or remove height by folding horizontally at the front - the opposite of the slash and spread at the center back seam.
For the sway-back adjustment:
1. Remove from the center back seat at waist, tapering to the crotch seam.
2. Add to center back seam, tapering to the side seams. This will compensate for what you remove at the center back and prevent you from getting a v-shape in the back waistband.
3. This isn't in the diagram, but darts can go a long way in helping you get the shape you need. Depending on the size of the alteration needed, it may look better to remove fabric at several points rather than all from the center back.