time to anchor: get your bank pins out, or your very long T pins.
Anchoring is done to prevent slippage, to position on grain and pattern, and to keep the fabric in the parameters of the space that piece is intented to cover. You anchor deeply, and well away from the seam lines ( inside them ). The doubled sections are self aligned, they should be atop one another in a matching way. You will see this happening, even though they are mirror image wrong side out.
As with any pins in fitting, you want to bury the point if you can. In anchoring, you may need to push them in and then out again. At the very least, point the points that do emerge toward the deck of chair.
The arm sections and front panel can be anchored first, then the excess brushed back over itself so that you can see all the inside and outside back. These pieces are cut big, there should be excess to brush out of your way. Pull taut on grain, but smooth is still the object, so do not pull on bias.
Your middle body sections get anchored onto the chair next. You stuck in pins previously to designate half way points. I am now going to tell you you may not use exactly that spot for your middle body fold placements. Why ?
Pull your fabric from side to side ( widthwise, warpwise, fillwise, whatever terms you use ). Do a test with 27". If you get more stretch than you are comfortable with, or familiar with, or no stretch at all, you may substitute your judgement for where the center fold belongs.
My comfort zone is 1/2" over the 27". If it pulls out about that much, I use the center point pins to designate where my folds are placed. If twice that ( one inch over 27" ) , I'll place my fold short of the half way pin by 1/8 of an inch ( or more ). Let me use different words to say the same thing. I will fold my outside back in half wrong side out, and place that fold short of my marker pins (to mark the half way) by an 1/8" on the side I am pin fitting . Why ?
It makes the cover smaller, it pulls out the excess stretch. 1/8" short is actually 1/4" removed. You can tolerate more than 1/8", but you should experiment in increments, smallest first. This applies to all three middle body pieces, the inside and outside back, and the deck.
If I had no stretch, I may steam goods to get some. If I still got no stretch having done that, I may place the fold OVER the half way point by 1/8" inch. That would make it 1/4" larger. Why am I doing all this ?
When you pin fit using other methods, you are pulling out excess elasticity, or allowing extra for ease, always. You just didn't mentally decide, you are manipulating with your hands, and it happens. Double on half is double, and you are pulling against a fold, and can only do so somewhat more meekly.
I have put fold 1/2" short or 1/2" over, but you need experience to make this adjustment this severe. If adding for inelastic fabric, you have no way to remove it once done, except to trim away at seam allowance on the outside back vertical seam. Even then, it's not shaped as perfectly had you not left this excess. If you placed the fold short of the half way point, then it's too small if you then needed more outside back and inside back. Err with deliberation, use a small number to begin with for this slipcover. Make more severe adjustments in the next slipcovers.
Anchor all before the next steps. All should overlap liberally. The next steps are slashing cuts to enable pivoting of fabric where it must turn, and closing seams. You will need to consider how to best make fabric turn the corner, as you may only have one chance to slash this.
You will close your seams in a hop skip and jump manner with the long pins on straight aways, and the shorter pins on curves. The seams belong right on the hard corners, and where ever excess comes together naturally and with ease, and to your eye, with style. Do not brush excess that is on one half of the seam only on down the line of pins, leave it where it occurs. Pin for darting or to leave as ease later. Do pin as if to dart. You need not dart this, you are just designating it " taken care of and noted ".
I work my entire chair when I hop skip and jump. I go from OB to FAP to DE to IB and Wing in no set order. I close first what is self evident, I postpone deciding what needs further clipping to shape untill I have closed up all else. I allow fabric to lie with ease where anchored, while pulling seams as tight as still looks at ease.
If you slash wrong, replace piece. You won't see this untill you SEE it, on your chair. Trim away if you are certain of seam position and the excess is in your sight line. Trim if and when you are familiar with consistent seam allowance. We will talk about trimming seam allowances next time. Leave your tuck areas alone for now, we will discuss this next as well.