okay, you are ready to clean up your pin fitting, make it safe to take off the chair, and have all the seams be recognizeable.
All the areas of the chair should be covered with your fabric, except the skirt area.
You can trim seams down on the chair if you wish, I think there's advanatage in doing that. You really get your nose in there and notice how the chair is shaped. You may err and cut too closely yes, but you have already clipped ( pivot cut or slash ) closely, and you have to seal with that anyway, so I'd say the risk is worth the learning.
While trimming, inspect the pins for straight lines of seams. Reinsert those that would give a false impression to the intended cut direction of seam. Fill in pins where the joined area is too flimsy to withstand being removed from chair intact. Remove anchor pins.
The deck has not yet been addressed. The deck piece ( or pieces ) must have a smooth appearance where it covers the chair front ( if it DOES ). If you plan a deck height skirt, this does not apply.
Attempt to depress the edge of the chair deck ( deck is where cushion sits ). I say attempt, it may be a hard board, and not move at all. If it moves as if there's a spring, you must fashion some of the inside arm or the deck piece ( or a blend of both ) to dive into this crevice. You may add a strip if the inside arm lacks the excess to do this. It is much easier to ask the inside arm ( and it's new strip ) to fill this crevice than to ask the deck piece to do so.
If you can manage the inside arm to dive into and back out of the crevice to the deck, then there's no darting needed. If you ask the deck piece to do this, a dart will show at the crevice front.
Eyes moving toward the back of the deck, the seat can be sewn to the insdie arms and the inside back. They need not be if you hem them and have tight tuck in spaces. I suggest you DO join them if the tuck in space is loose, and if the area in the tuck is big.
Depress the deck at all it's edges where it meets the inside back and arm. Are there any areas you cannot cram excess into ? Take note. Otherwise, and I trust you to do this as best you can, leave a four or five inch border around the deck space. It can be more, less. You should have excess at the inside back ( at bottom ) and inside arm ( at bottom ) to join to this border. Pin so as to have sewing instructions for yourself. Maybe it won't be as smooth as the rest of the pin fitting. It's not seen.
There may or may not be a vertical tuck seam where arm joins inside back. If you waited to close this area untill now ( or even if you inadvertently closed it with pins and trimmed it ) stick hand in there to see if tuck space is available. If so, and you choose to use this area for additional tucking, leave the same border as we did on the deck ( or add it back in a strip if you cut it off ).
Mark for intended skirt length. Make marks with pencil at a uniform distance from floor to the body where it will attach. Note that the mark needs to be 1/2" shy of the intended finshed length, to allow a seam. Skirt can start no higher up than the deck, that's a constraint. Skirt should not start below an existing skirt ( there would be a bump ). Skirt should not start below the upholstered body of a nonskirted upholstered piece. Skirt finished length need not hit the floor, but hold the ruler there anyway to make the marks, then do the note and the math as needed. Slipcover skirts must all be the same length, not shorter in back as soom upholstered pieces are. If you intend no skirt, rub the upholstery line with chalk, and cut away excess, leave a full inch seam there.
Next post will be preparation to unpin. The pin fitted cover should lift off the chair safely now. Pull it off. If needed pins fall out, replace them properly, quickly.