There are 3 main fishing techniques that we use in an outdoor survival situation. They are all very simple if you know them.
Find a thick and strong branch and cut it down to size removing the branches. About 1 foot long is a good length. Sharpen one end so it drives into the earth easier. Tie a line to one end. The line doesn't have to be fishing line, just any kind of rope, twine or paracord you can find. Tie the other end of the line to a rock. Working from the rock backwards, tie on lengths of fishing line about 1 foot apart and at varying lengths in order to attract fish at different depths. You can load the hooks with bugs, worms or flies then put the rock into the water. Leave the line for a good few hours whilst you tend to other activities, checking a couple of times a day.
The Lazy Man's line
This technique has numerous names but basically it's very similar to the nightline. With your peg in the ground, you'll need a longer line for this. This time, instead of the line attaching directly to the branch, it should thread through a hook, or trigger and up to a bent down branch from a nearby tree. The idea is, when the bait is taken, the trigger mechanism will release the tension on the line with the bent branch and the catch will be whipped out of the water and suspended away from anyone else nearby who may be hungry
The Looped line
This technique involves having 2 pegs in the ground at either end of a river bank. You make a single loop from your main line (not the fishing line) which is long enough to thread around both pegs whilst still having enough slack to drift out into the water. You can keep the hook lines at a relatively similar length for this which makes threading the loop around a little easier. This involves drawing the main line in from one direction whilst feeding it out in the other - rotating the loop. You would do this only occasionally as with the other techniques but the addition of many more hook lines gives this method a slightly better chance of getting a bite.