Hook up charger to battery and give it a good charge.

Depress throttle to 1/3-1/2 way down and hold it there.

Crank engine over for approx 1 minute, give the starter a break, and then repeat the crank period. Usually the engine will start on the 3rd starter cranking period. When you give starter a rest you can release the throttle but depress it before you turn on key as this seems to speed starting.

After engine starts you should go for a nice drive and do a little of the "Italian tuneup" driving as prescribed by Volvo.

This is 4 or more minutes of over 4000 rpm driving to cause valves to rotate so stems will clean themselves. There is not necessarily a need to remove and do anything with spark plugs.

You did exactly the actions in what has been labelled the "lawnmower syndrome".

Pull car out of garage, get out lawnmower, use it, put it away, and pull car back in garage. Car now attempts to start on next day and then refuses to start when valves don'r fully close. The condensation on exh valve stems makes a goo out of the deposits on stems and the hydraulic tappets will hold open valves due to tappets being able to fill during the time the gooed up valves spend off seat as they sluggishly move closed instead of following the camshaft lobes as they rotate.