Making crochet bags is a wonderful way to use up left over yarn. The pattern can be simple horizontal stripes using colors in random - or in a planned sequence.
Here are a few very easy pattern suggestions:
1. The plain double stitch
2. The plain treble
3. The shell pattern

Abbreviations:
dc- double crochet
tr – treble crochet
y.o.h – yarn over hook

1. The Plain Double crochet - dc:
Stitch description: insert hook into the 2nd chain from hook, * draw loop through, y.o.h, draw loop through both loops on hook. Insert hook into the next chain, repeat from * till the end, 1 chain, turn.
Over the required number of foundation chains, work -
Row 1: 1 dc into the 2nd chain from hook, work dc into each chain till end, 1 chain turn.
Row 2: 1 dc into 1st dc, 1 dc into each of the dc below till the end, 1 chain, turn. Repeat row 2 to the required length.
This stitch gives a close finish – you don’t have to worry about things dropping out of the bag.
Tip: This stitch works well as a grid for cross-stitch. If you have used single coloured yarn, spruce it up with some cross-stitch in different colours.

2. The Plain Treble crochet - tr:
Stitch Description: y.o.h, insert hook into the 4th chain from hook, *draw loop through, y.o.h, draw loop through the first 2 loops on hook, y.o.h, draw loop through the next 2 loops; y.o.h, insert hook into the next chain, repeat from * till the end, 3 chains turn.
Over the required number of chains, work -
Row 1: 1 tr into the 4th chain from hook, 1 tr into each chain till end, 3 chains turn.
Row 2: Skip 1st tr, 1 tr into each tr below until end, 3 ch, turn. Rep row 2 to the required length.
You can work this pattern faster than the plain dc pattern – but small objects like pens may drop out – you can use a lining while making it up into a bag.

3. Shell pattern: Chains required = Multiple of 6 + 1+ 1 turning chain.
Make the required number of chains; 1 turning chain, turn.
Row 1: 1 dc into 2nd chain from hook, * skip 2 chains, (5 tr into next chain) shell, skip 2 chains, into next chain work 1 dc: repeat from * until last 6 chains are left, skip 2 chains, 1 shell, skip 2 chains and into next chain work 1 dc, 3 chains turn.
Row 2: Work 2 tr into the dc below, * 1 dc into the 3rd tr of shell, shell in the next dc, repeat from * till last dc, 3 tr in last dc, 1 chain, turn.
Row 3: 1 dc in 1st tr, * shell in next dc, 1 dc into 3rd tr of shell repeat from * till last 3 tr are left, 1 dc in 3rd tr, 1 chain turn.
Repeat rows 2 and 3 for a beautiful shell pattern.