These are without a doubt one of the best ways to increase grip strength. Grab the handles in the middle and stand upright – do not lean forward! Stay upright. This will work your core as well as develop grip strength.
Keep your upper back tight and bring those shoulder blades together while externally rotating the shoulders, as if trying to show off your chest. As you walk, imagine that your upper body is one big block of iron!
Don't go as fast as possible. What we want is time under tension while moving a heavy load and keeping everything tight.
When doing loaded carries I often use the 10 meters = 1 rep approach. So if you're training for strength – which would involve doing sets of 1-5 reps on big lifts – it would translate to sets of 10-50m on the farmer's walk, with an optimal distance of at least 30 meters.