Step 1: Material

Material list:

flat lithium battery (e.g. taken from old mobile phone) (I used battery for Huawei Ideos X5 U8800)
Li-ion charger module with TP4056 (see on eBay)
DC-to-DC step-up module, with V_out = 5 V (see this one on eBay or better one here on eBay)
switch
Arduino Proto Shield or something equivalent
red and black wires (I used AWG24)
double sided tape

Optionally:

metal paperclips
some kind of lock connector (I used PSH02-02PG)

Step 2: About the Modules

Li-ion charger module with TP4056

See the figure above. This module contains two parts: charger (IC TP4056) and protection, which protect a cell against over-charging, over-discharging and against short circuit. Over-charging and short circuit may cause an explosion (see on YouTube).

Make sure you connect battery plus to "B+" and battery minus to "B-" of the module, because wrong polarity will destroy the protection circuit.

The R3 is resistor for setting maximum charging current.

R3 → max charging current

1.2 kΩ → 1000 mA (default)

2 kΩ → 580 mA

3 kΩ → 400 mA

For more details see TP4056 datasheet.

And read article about lithium cells on Wikipedia.

DC-to-DC step-up module

Classic step-up converter. Output voltage is constant 5 V. For more details see article about boost converter on Wikipedia .

I bought version with a USB connector dedicated for small DIY powerbanks. But desoldering the connector is redundant and not pleasurable work, so you should buy rather version without the USB connector.

Step 3: Wiring and Soldering

Make wiring according to schematic on the figure above. It shouldn't be hard.

As I wrote above, make sure you connect battery plus to "B+" and battery minus to "B-" of the charger module.

Don't attach the modules to battery with a hot glue, because they can heat up to high temperature and melt down the hot glue. Use a double-sided tape instead of the hot glue. Switch and connector you can attach with hot glue (as usual).

Step 4: Shield

If you complete previous step, you have standalone battery module now. It can be used in many different projects. And that's the reason why I don't want to attach the battery module to a proto shield permanently with a glue. In my case the battery module is attached to the shield just by 'helping holders'. These 'holders' are made from bended metal paperclips and soldered to my 'dummy' proto shield. See figures above. It works pretty good.