The following applies considering that eastern or semi-western grip for dominant hand is used on forehand and eastern or semi-western grip for non-dominant hand is used on the two-handed backhand with dominant hand at continental grip.
For top spin stroke it would be absolutely helpful making the task much easier, if the racquet face is turned at least perpendicular to the net or oncoming ball as soon as possible. It can be turned even a little more, but within reasonable margins. Also, on the forehand side the racquet with turned face better be in both hands and just behind the torso that combines the benefits of being ready for the swing and not too awkward for the 2nd Law of “Racing to the Ball.” Left hand up at the wide part of the neck/V-fork. One of the many terms for that particular position that I use is “Racquet Resting on the Left Hand” for righties. For lefties it would be right hand, obviously.
Eventually depending on time availability, when player bends knees and settles for the swing, hands will get separated forming L-Shape split position prior to the racquet motion forward to the ball. Non dominant hand stays on the striking side of the ball somewhat parallel to the net, while the dominant hand extends to some degree backwards.
On the backhand side I like to have non-dominant hand slide down all the way to the dominant hand and have the racket handle point towards the pants/shorts pocket with the face perpendicular to the net or even turned slightly “deeper.” I like to call it “Handle to the Pocket Position.” This way racquet, the same way as on the forehand, is ready for swing forward and, yet, comfortable for athletic movement to the random ball. Same as on the forehand side, when player bends knees and settles for the swing, hands will be taken slightly more back or “deeper” back for more energy, depending on time availability, as shown on the photo.
These are the two “Beginning of the Swing” responses that I have the most teaching success with.
Photos are used from the following web sites: