Usually at night, with thorough planning, backed up by overwhelming and merciless violence.

In a raid where prisoners were to be taken, they wouldn't be fooled or lured out of their trench; the assaulting party would enter the enemy line, kill the majority of the occupants in that section of trench and grab a few survivors to drag back with them, often severely beaten. A prisoner who didn't comply would simply be killed. The aim was to move quickly and completely dominate the enemy in the firebays being raided, then get out before reinforcements could arrive. Hand grenades and phosphorous bombs would be slung down into dug-outs (where most of the enemy would be resting). The raiders would be armed with revolvers, bayonets, daggers, and all manner of field-made 'trench clubs' (the Royal Engineers made a particularly nasty mace from a large metal cog welded on to a short length of pole) - the defenders, meanwhile, would likely just have their rifle, much less handy in such a fight. And their retreat might be covered by a rearguard party with a light machine gun and rifle grenades, or pre-registered trench mortar shoots on the enemy line just vacated.

In the British Army regular raiding activity was demanded from battalions in the line by GHQ, both as a means of intelligence gathering and a way to keep up 'fighting spirit' in troops that were otherwise mostly static (Haig referred to raids as "Winter Sports" in a diary entry of January 1916). They varied wildly in size, involving anything from a few men to many hundreds:

The German trenches have been entered by patrols of from 2 to 8 men, and for the purpose of securing an identification or obtaining information on some definite point, e.g. the existence of a suspected mine shaft... Raids on the enemy's trenches have been made by parties varying from 80 to 800 men, either with or without the co-operation of artillery.

Notes on Minor Enterprises issued by General Staff GHQ, March 1916

GHQ advice for the smaller raids of the type you mention was that the officers and men involved should be volunteers, and that they should be quartered together in comfortable billets for the week preceding the operation. All information regarding the point to be attacked would be carefully studied, and the target area would be kept under close observation to work out the enemy's patrol beats, listening posts, times of reliefs and so on. Practice advances would be made on mocked up wire and trenches closely resembling the target, and each man would be detailed for a specific job. The raid would take place at night, and the men would painstakingly crawl across no-man's land, quietly cutting through the barbed wire and remaining unseen and unheard until they suddenly dropped into the target trench. If surprise was lost the raid would usually be abandoned.

Some of the tasks to be achieved on raids were (direct source again is the GHQ Notes of March 1916):

To look for gas cylinders (carrying ropes to drag them back with if found).

To look for mine shafts

To take prisoners or secure identification

To destroy machine guns

Prisoners were a real prize, but the main goal on small raids was often just identification of the enemy unit occupying the trench: taking a German soldier's identity disc or pay-book was enough to achieve this. The much bigger raids by hundreds of men would be more likely to generate prisoners, but these would also usually take place with artillery and machine gun support suppressing the surrounding enemy.