The pressure needs to be on the sides of the penis, so you are doing it right.
With the overhand OK grip, the soft area between thumb and forefinger is on top of the penis and the thumb, forefinger and middle finger are working in opposition to each other against the sides of the penis. Don’t put pressure on the bottom.
With the standard OK grip it can be a little harder to turn the wrist enough for the soft area between thumb and forefinger to be right at the bottom, so you might be a little more towards the diagonal than the very vertical overhand OK but actually you probably want to leave a little space at the bottom to allow the sideways pressure to be exerted well. That is to say, don’t push up to the bottom of the penis with great pressure, make it loose and allow the pressure from the sides to deform the cross-sectional shape of the penis but even with a full pressure grip, try to avoid pressure on the bottom.
With regard to keeping the pump going, sometimes you lose sufficient erection and that’s OK, just take a little time out to recover your erection. If the adverse is the problem and you are too erect you also won’t find that pressure on the side does the job, so allow time for your erection to flag a little. You need that goldilocks level of erection, not too much and not too little.