3. Spend time on design and a detailed drawing, solving every little issue in advance. Check your design with positions of  electrical outlets, lights, heating, water pipes etc.
How much overhang do you prefer? 10 degrees is too little, 45 is too much. A steeper wall will be more like a floor to construct. Don't underestimate the added weight and workload associated with anything steeper than maybe 30 degrees. Anyway, for each sheet of 2400 mm plywood, 5 studs (= joists, framing bars) from 45x90 mm lumber seems to be standard here and now for a 25 degree wall (given that the sheets are landscape/horziontal and not portrait/vertical) 

Now simplify your construction to reduce material, work and cost by at least 50 percent. Horizontal or vertical panels? Will there be any leftovers? Do you really need  T-nuts all the way down?

4. Make a shopping list of everything you fail to find second hand.
Compare prices of a few suburban construction markets. Negotiate prices and make your choice. Be careful not to accept any lumber that is not completely straight (this will disqualify about 50 percent of what you find in the shelf), else the plywood won't fit.

5. Find out the exact locations of supporting structures in roof and walls, if not concrete. Use a stud finder, or drill a 1 mm hole from above or behind for examination to know exactly where to attach your contstruction.
Mark the exact postitions of kicker panel and roof stud on floor, walls and ceiling using tape.

6. Assemble a kicker panel (a vertical bottom section required for steeper walls) from 2 horizontal studs and 5 binders, joined by single vertical framing screws. Put it in position. You might not need to attach it to the wall.

7. Attach a stud to the ceiling. You will need some assistance here. Use another pair of studs to squeeze it in place while working.

8. Roughly cut a first joist, hold it in place and use a pencil to mark the exact cuts to make it join kicker panel with roof stud. You may have to make a second cut in both ends to make it fit, depending on the wall angle of your design.

9. Cut the first joist, confirm and adjust.

10. Mark and cut the remaining joists, using the measures of the first but adjusting for irregularities in lumber, floor, walls and roof.

11. Attach the joists using a vertical framing screw, supported by another one inclined from the side of the joist. Joist hangers (balkskor) might be overkill for anything overhanging less than 30 degrees, depending on the supporting structures of ceiling and walls. Check every position for square (else the plywood won't fit).

12. Now measure the actual length needed for each horizontal joist blocking (kortling). Cut and attach, with double horizontal framing screws where possible. Assemble with the screws inclined (skråspikning) where the end of a stud is blocked by another.

It is easier to attach the horizontal blocking in one piece behind the vertical joists, but then the seam between the plywood sheets will be unsupported. 
On the other hand, more than a top and bottom blocking per 1200 mm horizontal/landscape plywood seems to be overkill.

13. Mark the positions of the T-nuts on one sheet of plywood, using tape measure and pencil. A grid of 160 mm squares might fit the sheets without  colliding with the framework. The kickboard will have no grips (maybe screw-on foot chips) and thus need no drilling.

14. Clamp the two plywood sheets together and start drilling. If these holes are anything less than perpendicular, then you won’t be able to attach grips. This cannot be fixed afterwards. Freehand drilling will hardly be accurate enough, get some kind of tool or support for a 90 degree angle.

15. Paint the plywood using a roller - while still on the ground.

16. Attach the T-nuts, using a hammer. Attach a pair of jug grips on each sheet to make sure the drilling was accurate enough and to make it easier to handle the sheets.

17. Attach the plywood sheets, using decking screws. No drilling needed. You need assistance to keep each sheet in position. Start with one screw in each supporting stud, then dismiss your assistant and add screws roughly every 200 mm.
With this basic design you don’t need to cut the plywood sheets, except for the kickboard.

 

18. Attach grips. Remember a steinpull (dry tooling undercling) puts an immense leverage on any grip. Footholds won’t be necessary as long as you accept to attack the wood with your front points. 

19. Provide for a safe landing. Cover it with something that will accept your crampons, such as a mat.

20. Get some company and start working out. Remember you now have to spend more time in this garage than in the gym to break even in a year…