Buy Burpless Tender Green F1 seeds.
Fill 3" plant pots with multipurpose compost, water first and drain. Push one seed in each pot, upright, with long edge horizontal and only just covered with compost. Scatter a 1/4" layer of vermiculite on top and place in aheated propagator/ airing cupboard / or a warm place indoors. After germination, which will only take a few days, bring into the light e.g. greenhouse bench and leave to grow. When they are larger - when roots are showing through base of pots - pot on into 5" pots, using more multipurpose compost, without disturbing the roots, and water.
When the plants are much larger and perhaps showing signs of flowers, pot on, into 12" pots, again with multipurpose compost,water and place in their final growing positions in the greenhouse. Provide support and care during growth according to the instructions on the seed packet.
Keep a bin full of water in the greenhouse, add the required amount of fertiliser e,g,tomato or miracle grow and use this for watering as it will be very handy and the same temperature as the plants. Only water when the compost feels dry to the touch, or if the weight of the pot feels lighter than usual, or if the leaves look limp. Do not over water. Cucumbers like water (they are mostly water) but too much can kill more easily than too little. Too little can be remedied but too much cannot.
Do not sow until it is warmer - April/May. It is easy to get seeds to germinate in warmth but after germination you need to be able to provide light and a warm situation outside the propagator. Good luck!